A drive bay is a standard-sized area for adding hardware to a computer. Most drive bays are fixed to the inside of a case, but some can be removed.

Over the years since the introduction of the IBM PC, it and its compatibles have had three types of drive bay, of which two are in common use today.

Types

Full-height

Full-height bays were found in old PCs in the early to mid-1980s. They were approximately 3.5" high by 5.75" wide, and used mainly for hard disks and floppy disk drives.

5.25"

Half-height or 5.25" drive bays are approximately 1.75" high by 5.75" wide, and are the standard housing for CD and DVD drives in modern computers, but were sometimes used for other things, including hard disks and floppy disk drives in the past. They used to be called half-height in reference to full-height bays, and are now better-known as 5.25" in comparison with 3.5" bays. It is important to note that the name does not refer to the width of the bay itself, but rather to the width of the disks used by the floppy drives which mounted in these bays. Often represented as 5¼-inch.

3.5"

3.5" bays, like their larger counterparts, are named for diskette dimensions; their actual dimensions are 4" wide by 1" high. Those with an opening in the front of the case are generally used for floppy or Zip drives. Hard drives in modern computers are typically mounted in fully internal 4" (nominally 3.5") bays. Of course, nowadays most computers - especially laptops - don't come with floppy drives at all since CD/DVD-RW drives are very common. There are adapters, sometimes called a "sled", which can be used to mount a 3.5" device in a 5.25" bay. Often represented as 3½-inch.

Usage

Drive bays are most commonly used to store disk drives, although they can also be used for front-end USB ports, I/O bays, card readers, fans, tool storage, and other uses. Some computers have a small system monitor LCD display mounted in a drive bay.

When installing a drive in a bay, it is usually secured with 4 screws that hold the drive in the bay, although toolless fasteners are becoming more common. Then, any necessary power, data transfer, and other cables are routed into and connected to the rear of the drive. The drive bay is usually just big enough for the drive to fit inside.