Formatted capacity
The smallest available hard drives (20GB or so) provide enough capacity for the typical business desktop, but you can often get two to three times that capacity at a price only 30 percent to 60 percent higher. Midrange models offer the lowest cost per gigabyte--plenty of cheap space for MP3 enthusiasts and other moderate consumers of digital media. The largest available drive (currently 100GB), which always carries a premium, will suit digital-video hobbyists and graphics professionals.

Interface
The type of connection between the hard drive and the system. Drives with EIDE (enhanced integrated drive electronics) interfaces dominate, with every desktop PC offering built-in EIDE connectors. Typically, only servers use SCSI (small computer system interface) drives, which cost much more and require an interface card but provide higher performance when multiple users access the same drive. Current EIDE drives generally conform to the ATA/100 specification (also known as Ultra ATA/100, Ultra DMA/100, and Feature ATA). The 100 in ATA/100 indicates that up to 100MB per second (MBps) can transfer from the drive to the system in short bursts. The prevailing SCSI specifications, Ultra160 and Ultra 320, support 160MBps and 320MBps (respectively) burst transfer rates.

Spindle speed
Expressed in repetitions per minute (rpm), this spec offers the best single clue to drive performance. Desktop drives generally come in 5,400rpm and 7,200rpm varieties, with 7,200rpm drives averaging 10 percent faster (and 10 to 30 percent more expensive) than 5,400rpm models. You can feel performance improvement. Therefore I recommend you go with 7200rpm. High-end 10,000rpm and 15,000rpm hard drives offer only marginally better performance than 7,200rpm drives--and cost much more, in part because they are typically SCSI drives with added reliability features.

Data transfer rates
The external transfer rate, also known as the burst transfer rate, is a relatively meaningless number. It refers to the top speed at which data can be transferred between the hard drive's cache memory and the system (interface specifications such as ATA/100 indicate the external transfer rate). The internal transfer rate, also termed the sustained transfer rate, tells more about the speed of the drive. Generally ranging from 14MBps to 62MBps, it indicates how fast data can be read from the outermost track of a hard drive's platter into the cache. A difference of a few megabytes on the high end of that range won't be perceptible, but specs on the low end may indicate a slow drive, particularly for such demanding applications as video editing.

Seek times
If two drives have the same spindle speed, you may be able to determine the faster drive be checking the seek time, which measures how long it takes on average for a hard drive's read/write head to find a random track. Small differences in seek times, which range from 3.9 milliseconds (ms) for ultrafast SCSI drives to 12.1ms for slower EIDE drives, may be noticeable in database or search applications where the head scoots all over the platter. Avoid drives with seek times that are more than 9.5ms.

Cache
The amount of memory built into the drive. Designed to reduce disk reads, the cache (also known as the buffer) holds a combination of the data most recently and most frequently read from disk. Cache memory sizes generally range from 512K to 4MB, but some high-end SCSI drives have 8MB or even 16MB. Large caches tend to produce greater performance benefits when multiple users access the same drive at once. Although small differences in cache size may have little bearing on performance, a cache smaller than 2MB may be a sign of an older, slower drive.

Warranty and support
The standard hard-drive warranty lasts three years, with some SCSI drive warranties stretching to five years. Only high-end drives come with free telephone support, but in all cases, frequently accessed support information describing jumper settings and so on will be found on the vendor's Web site.