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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Configuration and Implementation | RAID Hard Disk Drive Requirements ]
When I talk about drive size in a RAID array, I really mean two different things: first, the capacity of the drives in gigabytes, and second, the physical size of the drives. The first is important to the overall capacity of the array; the second has important implementation implications (gotta love that alliteration. :^) ) And the two are related as well, because larger drives are often physically larger than small ones.
The capacity of the drives used in a RAID array affects the capacity of the overall array, of course. Larger drives yield larger overall capacity for the array, all else being equal. For this reason, RAID arrays tend to be constructed of large drives whenever possible. This is especially true because the cost of hard drives per gigabyte decreases when you buy the larger sizes of any given family. Another important issue is that drives should all be the same capacity when used in a RAID array, or you will forfeit any additional capacity on the larger drives in the array. For more on these and other capacity issues, see here.
If you have a limited budget, you will be forced to some extent to trade off individual drive capacity against the number of drives you purchase; this is the reason for the "should I buy more small drives or fewer large" drives conundrum examined here. More drives means better performance, but fewer large drives can be substantially less expensive. If you need large capacity but are limited by physical considerations to a small number of drives, obviously, get large drives. Another reason to consider large drives besides cost and space is expansion. If your system can handle a total of six drives and you fill all six spots with small drives, you'll need to replace drives--possibly all of them--if you want to increase capacity later on (which sometimes is the best idea anyway, but can be expensive and time-consuming; see here.)
Physical size is another issue, more specifically the form factor of the drives. The standard hard disks used today in RAID configurations are almost always 3.5" form factor drives, which describes a standard width and depth for these units. There are two different heights found in 3.5" form factor drives, however. The standard height, called slimline or low-profile, is used for all standard IDE/ATA and most smaller SCSI hard disks; these drives are 1" high. For high-end SCSI drives however, the largest member of most families use anywhere from 6 to 12 platters, too large to fit in a 1" package; these drives are 1.6" in height and are called half-height drives (the "half" being relative to the full size of the ancient 5.25" floppy drive bay in the original PC--don't ask. :^) ) The hard disk shown on the main page of the hard disk reference section is a 1.6" "half-height" drive.
Most regular PCs are not designed to take these larger form factor drives, because their drive bays are sized for low profile drives. Some may still take 1.6" drives, especially if there are two contiguous 3.5" bays without obstructions between them. Still, if you really want to use half-height drives you want to use a server case that is designed specifically for these larger devices. This will also help avoid cooling and power problems when using several larger drives.
Next: Drive Selection Criteria