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Hard Drives Buyers Guide by Sean Cleveland The elements of a drive How data is written Areal density Magneto-resistive heads Spindle speed Internal and external transfer rates Internal cache buffer S.M.A.R.T. Interfaces compared IDE and ATA DMA and bus mastering UltraATA/DMA technology CRC error checking SCSI Ultra2 SCSI LVD (low-voltage differential) Ultra160 SCSI Single-ended (SE) signaling High-voltage differential (HVD) Comparing IDE/ATA and SCSI Benchmarking Average read/write speed Seek speeds Random access time Read burst speed CPU utilization Price range Start shopping	Shop For: Cheap CD-RW Recordable Drives Cheap DVD Recordable Drives Cheap Hard Drives

Buying a hard drive these days is no easy task. With so many types to choose from, multiple connection interfaces, and specifications becoming more difficult, it's a wonder hard drives are even sold individually. Our buying guide puts all those concerns and frustrations aside as we break down the technical barriers and explain hard drives in simple English. Table of Contents
The elements of a drive A basic understanding of hard drives' internal workings is essential for understanding and putting the various technical terms in perspective. We've provided some background that explains just how data flows from a hard drive into your machine. How data is written A hard drive contains several round, flat disks called platters. Platters have a coating that allows data to be written to them magnetically. These platters are stacked on a spindle with just enough space between them for an instrument called a read/write head to pass between them. These heads use electromagnetic signals to read and write data onto the platters. Most drives use two heads per platter, one above and one below. The heads write the data in concentric circles called tracks. Tracks contain a number of sectors, which can each hold 512 bytes of data. Areal density Areal density is the measurement representing the amount of information that can be packed into a physical space. For hard drives, the measurement is given in bits per inch (BPI). Typically, it's given in millions (megabits) or billions (gigabits) per square inch. Since areal density is the one hard-drive specification that is increasing at the fastest rate, the measurement, which was once quoted in megabits per square inch (Mb/sq.inch), is now quoted in gigabits per square inch (Gb/sq.inch). The areal density of a drive is important as it helps determine how old a drive is and what kind of transfer rate you can expect. A higher areal density means the bits are packed tighter, which directly affects the speed of the drive. Drive heads can read more data faster when the data is packed into tighter spaces, since the hard drive motor spins the spindle and the disks at a constant speed (described below under spindle speed). Therefore, drive innovations that increase the areal density directly affect the speed at which data can be read from the drive and transferred to your machine. Ultimately, areal density determines the amount of gigabytes (GB) that can be stored on a single hard-drive disk (both sides). The areal density multiplied by the total number of disks used in the drive determines the full size of the drive. This is how drive manufacturers make drives of varying sizes: they build them with different numbers of disks. Magneto-resistive and giant magneto-resistive heads Magneto-resistive, or MR heads, use separate read and write elements that are optimized for their individual capabilities. The read element is composed of a thin-film material. When it's near a magnetic field, this material changes in resistance to produce a stronger signal that allows for greater areal densities and larger disk capacities. The continued advancements in MR heads has produced giant magneto-resistive (GMR) head technologies that allow for the miniaturization of drive heads that can read smaller pieces of data faster, directly affecting overall read/write performance of the hard drive. Spindle speed Spindle speed is the speed at which the hard disks spin. The motor that spins the hard disks is usually a direct drive motor, and the motor's shaft is called the spindle. The spindle speed is considered the mechanical speed of the drive; the faster the speed, the faster you can access and read data. When a drive's specs show it has a 7,200 RPM spindle speed, it means the disks are spinning at 7,200 rotations per minute. Currently, the lowest spindle speeds run at 5,400 RPM; these speeds are meant for budget machines and users who covet reliability over performance. Drives that run at 5,400 RPM are considered more stable and reliable as they don't contain high-performance parts. They're also designed to be larger in size to target data-hungry consumers. Hard drives with spindle speeds of 7,200 RPM fall into two separate classes. First, there are ATA hard drives, which are built to be used on the IDE bus with 7,200 RPM drives. These are considered high-performance drives, as 7,200 RPM is the fastest speed these drives offer. Generally, manufacturers build 7,200 RPM ATA hard drives for performance; the ATA hard drive should be the main drive in the system and the one you install your operating system on. The second class of hard drives are SCSI hard drives with 7,200 RPM speeds, which are on the low end of the SCSI spectrum. High-performance SCSI hard drives have spindle speeds of 10,000 RPM and generally contain large cache buffers and high-performance SCSI connectors. However, these speeds are starting to look slow as faster drives with speeds between 12,000 and 15,000 RPM emerge. Internal and external transfer rates The internal transfer rate is the speed at which the heads can read data from the platters and pass that data to the drive's internal cache or read buffer. The external transfer rate is the speed at which the heads can send the data from the read buffer across the drive's interface and across the cable to your machine's memory. The external transfer rate is also referred to as the interface transfer rate or burst transfer rate. This is the fastest rate and the one most drive manufacturers like to quote. It's called burst, as data being sent across the cable from onboard memory can easily fill the whole bus, regardless of the interface. Internal cache buffer The internal cache buffer is the memory that's inside the hard drive and that bridges data between the drive and the interface. This cache memory is much faster than the mechanical drive. Cache memory stores data that the drive has previously read, so when you request data, your system checks the cache first before checking the actual drive. As you can guess, it's best to read data from the cache buffer, as data taken from this area travels at the fastest speed. However, the sustained transfer rate of the drive is not fast enough to fill the onboard cache for the system to use. When this happens, the system bypasses the cache and looks for the data directly on the drive's disks, lowering the data transfer rate to the slower, internal transfer rate. This is how your system accesses most larger pieces of data. S.M.A.R.T. Built into hard drives or computers, S.M.A.R.T., or self-monitoring analysis and reporting technology, monitors the drive for potential problems during predictable failures. S.M.A.R.T. uses the parameters set by the drive manufacturer, and both the motherboard BIOS and drive itself must support the technology. Interfaces compared You've probably seen the terms IDE, ATA, and SCSI (pronounced "scuzzy"), but you might not know what they mean or what the advantages of each are. Here we explain what the acronyms mean, break down the technologies, and tell you which interface is best for different users. IDE and ATA IDE, or integrated drive electronics, refers to the controller on the hard drive itself. ATA stands for AT attachment and refers to the interface that an IDE-based hard drive uses to attach to a computer. (AT was the name of IBM's first 286-based PC and stands for Advanced Technology.) Generally, manufacturers refer to hard drives by the attachment interface they contain (i.e., SCSI hard drive, ATA hard drive). This is why you should refer to IDE-based hard drives as ATA hard drives. DMA and bus mastering DMA (direct memory access), or bus mastering, is support that allows data transfer between the hard drive and memory without using the CPU. Both the hard drive and IDE controller in the machine on the motherboard must include support for DMA. Also, you must manually enable the operating system itself to support DMA--this is done via the System section in the Control Panel in Windows 95/98. Enabling DMA transfers greatly improves the speed of the hard drive, but if problems occur after it's enabled, we recommend that you promptly disable it. UltraATA/DMA technology UltraATA technology was invented by Quantum (along with UltraSCSI, Ultra2, and Ultra160 SCSI technologies). UltraATA refers to the actual hard drive that supports the UltraDMA bus on the motherboard or add-in controller. Currently, there are two flavors of UltraATA technology: UltraATA/33 and UltraATA/66. UltraATA/33 was the first UltraATA spec introduced, and this version increased the maximum transfer rate of the IDE bus from 16 MB per second to 33 MB per second. The UltraATA/33 accomplished this by transporting twice as much data per clock cycle. In addition, the UltraATA/33 reduced CPU overhead even more and improved timing margins, while a CRC (cyclical redundancy check) detected bad data and provided data-protection verification. UltraATA/66, Quantum's most recent technology, doubles the previous total bus bandwidth to 66 MB per second and extends ATA-based technology to at least the year 2001. Previously, UltraATA/33 doubled the transfer rate by keeping the clock rate the same but transferring data on both sides of the clock--called double-edged clocking. UltraATA/66 doubles UltraATA/33's rate by changing the set-up and hold times. Before the controller can transmit data across the cable, it must first wait for the spikes and electronic noise interference on the cables (called crosstalk) to die down. The problem is the controller can mistake any remaining interference for data. To remedy this problem, Quantum introduced a timing margin in the UltraATA/66 spec that inserts a period of time between transfers. The timing margin provides enough time for interference to fade away before the controller starts detecting for ones or zeros (or real data) again. You'll need a special cable to get UltraATA/66 hard drives to work properly with UltraATA/66 controllers or motherboards that support UltraATA/66 drives. This new cable contains 80 conductor lines but still only uses 40 pins on each connector. The new lines are additional ground lines, one for each data line for a total of 80 lines. (Of the 40 lines in the older cable, only seven contained ground lines.) If you use an older cable (40 pins and 40 lines/wires), the drive will still function but you'll limit drive speeds and data bursts from the onboard cache buffer across the bus to 33-MB-per-second speeds instead of utilizing the full 66 MB per second that the drive and bus deliver. (See our section on CRC error checking for more information.) The third generation of the Quantum-developed UltraATA spec is due at the end of 2000 and will increase the bus to 99 MB per second. This new spec will officially be called UltraATA/100. It's expected that UltraATA technology will end after that to be replaced by Serial ATA, which is designed to last up to 10 years. The Serial ATA group includes Intel, Dell, IBM, Maxtor, Quantum, Seagate, Western Digital. Serial ATA, comprised of much smaller and clutter-free cables that don't impede airflow, is based on serial technology where data is transferred one bit at a time instead of being transmitted over several wires simultaneously (such as the current ATA architecture, which is now being referred to as Parallel ATA). The benefit is a smaller four-pin cable (down from the current cables that use 40 pins) and the ability to support transfer rates of 150 MB per second in the first release (called Serial ATA 1X). Subsequent releases of Serial ATA will double throughput speeds. CRC error checking Because of the high data speeds, it is important to use CRC error checking for data integrity. Going forward, motherboard chipsets that support UltraATA/66 will enable you to detect whether you're using a 40-conductor cable or 80-conductor cable to prevent UltraATA/66 drives from working at full speeds across older 40-conductor cables. Chipsets accomplish this check by detecting whether one of the lines has a break in it--older 40-conductor cables do not contain this break. If the chipset doesn't detect a break, the drive will only operate at 33 MB per second. SCSI SCSI, or small computer system interface, is a complete expansion bus you can use to connect a wide variety of peripherals to your computer--the most popular being hard drives. Every SCSI hard drive requires you to use a dedicated SCSI controller card, which are often sold separately from the drive. There are many different varieties of SCSI, and we explain the most common below. Ultra2 SCSI Ultra2 SCSI doubles the total bus throughput of Ultra Wide SCSI (40 MB per second) to 80 MB per second by increasing the clock rate to 40 MHz. The Ultra2 also uses a new type of transceiver called low-voltage differential (LVD). (See our section on LVD.) The transceiver dictates the overall cable length and the total number of devices that the Ultra2 SCSI bus can support. LVD (low-voltage differential) Low voltage differential (LVD) signaling draws dramatically less power, has a bus length of 12 meters, and is backward compatible with older SCSI devices of every flavor. For LVD to work properly, you must use a special cable with an LVD terminator to connect Ultra2 hard drives to the Ultra2 SCSI host adapter. Ultra160 SCSI Ultra160 is the latest flavor of SCSI. It doubles the bus speed of Ultra2 SCSI from 80 MB per second to 160 MB per second. Ultra160 does this by using a new feature called double-edge clocking that doubles the transfer rate by taking advantage of both the rising and falling edges of the REQuest/ACKnowledge clock. Since the Ultra160 accomplishes this without additional internal clocking, it reduces incompatibilities with older SCSI devices. This drive also includes CRC (cyclical redundancy checking) for data integrity and uses the same LVD cabling as Ultra2 SCSI. Single-ended (SE) signaling The most popular SCSI signaling technology is single-ended (SE) signaling. This sends each signal over a single wire with a second wire for ground. It offers a bus length of 3 meters and generates a lot of line noise, which is why it's only used for slower SCSI devices, such as CD/DVD drives and slower SCSI tape backup devices. High-voltage differential (HVD) signaling High-voltage differential (HVD) signaling offers bus lengths of up to 25 meters, although it is not electrically compatible with SE. (It can't control slower devices such as tape backup drives and CD/DVD-ROM devices.) HVD signals are sent on two wires with the data represented as the difference in voltage between the wires. Comparing IDE/ATA and SCSI There's a lot of confusion between ATA and SCSI drives as to which you should use where and when. Here's a quick rundown of the benefits and detriments of both technologies. ATA's biggest shortcoming is its limited cable length. It's currently only 18 inches long, which makes it unsuitable for external devices. This fact alone forecasts its eventual replacement by interfaces such as 1394 (FireWire) or USB. But ATA technology is so commonplace in PCs, it won't be disappearing soon. With UltraATA technology increasing and UltraATA/100 and Serial ATA technologies just on the horizon, the future of ATA is secure. The technology already performs somewhat on par with SCSI drives on the desktop, especially in non-multitasking operating systems, such as Windows 95/98. High-end SCSI hard drives are the staple of power users, high-end workstations, and servers for the myriad features IDE doesn't contain. SCSI has a fat data bus backed by features that facilitate the movement of data, such as its ability to execute up to 256 simultaneous threads of data compared to IDE's single-threaded I/O interface, which can only execute one I/O request at a time. Ultra DMA, the hero of the ATA/IDE world, is still limited by these drawbacks and only doubles the data bus while adding a few enhancements, such as data integrity. High-end SCSI just got a shot in the arm with the Ultra160 interface that increases the bus throughput to 160 MB per second--far faster than UltraATA/66's 66 MB per second. The downside of SCSI is that it costs more and requires a separate dedicated SCSI host adapter. It's also a bit more difficult to install. But don't be scared of SCSI--it's not that tough to configure, and the payoff is worth the effort. Video professionals covet SCSI drives for their high throughput, low CPU overhead, and smooth transfer rates. And with video hardware costs finally coming down into the consumer range, SCSI kits and drives are becoming easier to install as their manuals are rewritten for the average consumer. Benchmarking Other elements besides internal and external transfer rates and spindle speed dictate a hard drive's overall performance rating. Just as important, and sometimes more, are write speeds, random access times, CPU utilization, and I/O rates, which are all explained below. Average read/write speed The average read/write speed is a sequential read and write. The drive reads the data files one after another in sequence, and measures the data at multiple locations on the disk drive to illustrate the varying speeds of the hard drive. Sequential speeds illustrate the raw-data-handling capacity of the drive. This is typically recorded in megabytes per second (MB/sec) and the higher the number, the better. Seek speeds Seek speeds are typically given by the manufacturer of the hard drive. Seek is the amount of time it takes to move the access arm that contains the read head to a specific track on the drive. Random access time, which we explain below, is more important. Random access time The random access time measures the time it takes the drive to read a random sector on the disc. This is different from the seek time that most manufacturers publish (the amount of time it takes to move the head to the proper requested track). Random access actually times the seek along with the time it takes to access non-sequential data. Random access time takes rotation latency into account and is a much more useful indicator of overall drive response. Recorded in average milliseconds, a lower score is better. Read burst speed Burst rate measures the maximum data rate between the drive's onboard cache buffer and the system memory. Also referred to as the drive's external data transfer rate, this is the fastest rate at which data can be sent across the cable to the machine. Many IDE systems will perform at acceptable levels only when you enable DMA. This is typically recorded in megabytes per second, and a higher number signifies a faster drive. CPU utilization The amount of CPU used while the drive is operating at full speed is very important. The higher the score, the less access you have to your machine during large reads and writes (like file copying). Enabling DMA greatly reduces the amount of stress on the CPU. This is typically recorded as the percentage of CPU used. Price range Competition in the hard-drive market is primarily based on pricing. Typically, the lowest priced drives are the slower 5,400 RPM drives, which are classified by size. The faster 7,200 RPM ATA drives are the best performing in this class and, as such, command the highest prices, which are determined by size as well. Drives with 5,400 RPM usually go for between $150 and $300, while faster and larger-sized 7,200 RPM drives go for $300 and more. SCSI drives are always more expensive due to their high-performance nature and the fact that SCSI-based drives have additional high-end chips. SCSI drives also require a high-end SCSI host adapter that plugs into a computer's PCI slot. This drives the price of SCSI even higher, generally putting it out of reach for the typical consumer. High-end SCSI drives can go for between $300 and $1,000, depending on their spindle speed, drive size, and type of connection technology. However, the home user can get by with an Ultra2 SCSI-based drive just fine. While buying older drives is certainly cheaper, it's generally best to buy ones that use the latest technologies, as they often perform better and contain innovations that help ensure stability, reliability, and longevity. Start shopping Think you're armed with enough information to start shopping? Check out all our Hard Drives now.