Disk Enclosure - Form Factors

Form Factors

• Multiple drives: RAID-enabled enclosures and iSCSI enclosures commonly hold multiple drives. High-end and server-oriented chassis are often built around 3.5-in drives in hot-swappable drive caddies.

• "5.25 inch" drive: (5.75 in × 8 in × 1.63 in = 146.1 mm × 203 mm × 41.4 mm)
  Most desktop models of drives for optical 120-mm disks (DVD-ROM or CD-ROM drives, CD or DVD burners), are designed to be mounted into a so-called "5.25-inch slot", which obtained its nickname because this slot size was initially used by drives for 5.25-inch-diameter (133 mm) floppy disks in the IBM PC AT. (The original "5.25-inch slot" in the IBM PC was with 3.25 in (82.6 mm) twice as high as the one commonly used today; in fact, the PC's drive size was called "5.25 inch full-height", and the size used in the PC AT and commonly used today is "5.25 inch half-height".)

• "3.5 inch" drive: (4 in × 5.75 in × 1 in = 101.6 mm × 146.05 mm × 25.4 mm)
  This smaller, 4-inch-wide (100 mm) disk-drive form factor was introduced with the Apple Macintosh series in 1984, and later adopted throughout the industry beginning widely with the IBM PS/2 series in 1987, which included drives of this size for 90-mm ("3.5-inch") floppy disks. This form factor is today used by most desktop hard drives. They usually have 10 mounting holes with American 6-32 UNC 2B threads: three on each side and four on the bottom.

• "2.5 inch" drive: (2.75 in × 3.945 in × 0.374 in = 69.85 mm × 100.2 mm × 9.5 mm)
  This even smaller, 2.75-inch-wide (70 mm) form factor is widely used today in notebook computers and similar small-footprint devices. One commonplace feature for these drives is radically lower power consumption than is found in larger drives. This enables enclosure vendors to power the devices directly from the host device's USB or other external bus, in most cases.

• "1.8 inch" drive: Found in extremely compact devices, such as certain portable media players and smaller notebooks, these devices are not standardized like their 2.5 inch cousins.

A range of other form factors has emerged for mobile devices. While laptop hard drives are today generally of the 9.5 mm high variant of the "2.5 inch" drive form factor, older laptops and notebooks had hard drives that varied in height, which can make it difficult to find a well-fitting chassis. Laptop optical drives require "slim" 5.25-in enclosures, since they have approximately half the thickness of their desktop counterparts, and most models use a special 50-pin connector that differs from the 40-pin connectors used on desktop ATA drives.

While they are less common now than they once were, it is also possible to purchase a drive chassis and mount that will convert a 3.5-in hard drive into a removable hard disk that can be plugged into and removed from a mounting bracket permanently installed in a desktop PC case. The mounting bracket carries the data bus and power connections over a proprietary connector, and converts back into the drive's native data bus format and power connections inside the drive's chassis.