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 many form factors of drive bays. Four form factors are in common use today, the 5.25", 3.5", 2.5" or 1.8" drive bays.
8.0″ drive bays were found in early IBM PCs and TRS-80 in 1979. They were 4.624″ high by 9.5″ wide, and approximately 14.25″ deep (117.5mm x 241.3mm x 362mm), used for hard disks and floppy disk drives. Other drive types such as CD-ROMs and other optical and magnetic drive were yet to be designed. These drives are obsolete.
Full-height bays were found in old PCs in the early to mid-1980s. They were 3¼″ high by 5¾″ wide, and up to 8″ deep (83 × 146 × 203 mm), used mainly for hard disks and floppy disk drives. Half-height drive bays are 1⅝″ high by 5¾″ wide (41.3 mm × 146.1 mm), 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. 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.
The dimensions of a 5.25″ floppy drive are specified in the SFF standard specifications SFF-8500 and SFF-8501 which were incorporated into the EIA-741 specification by the Electronic Industries Association (EIA). Dimensions of 5.25 optical drives are specified in the SFF standard SFF-8551 (they're somewhat shorter and not only size of the body is standartized, but also size of the bezel).
3.5″ bays, like their larger counterparts, are named after diskette dimensions; their actual dimensions are 4″ wide by 1″ high (102 mm × 25 mm). 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. They are limited to 0.7″ (18 mm) height to allow room for airflow. Of course, most of today's 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.
The dimensions of a 3.5″ drive are specified in the SFF standard specifications SFF-8300 and SFF-8301 which were incorporated into the EIA-740 specification by the Electronic Industries Association (EIA).
2.5″ bays, actual dimensions are 2.75″ wide by 0.197-0.750″ high and 3.955' deep (69.85mm x 5-19.05mm x 100.45mm). However most laptops have drive bays smaller than the 15mm specification. 2.5' Hard drives may range from 7mm to 15mm in height, there are two sizes that appear to be prominent. 9.5-10mm size drives are predominately used by laptop manufacturers, however at present some higher capacity drives (above 1 TB), are 15mm in height. The greater height of the 15mm drives allow more platters and therefore greater data capacities. Many laptop drive bays are designed to be removable trays in which the drives are mounted for easy removal and replacement.
The dimensions of a 2.5″ drive are specified in the SFF standard specifications SFF-8200 and SFF-8201 which were incorporated into the EIA-720 specification by the Electronic Industries Association (EIA).
1.8″ bays have two specifications, a (60mm x 70mm) form factor and a (54mm x 78mm) form factor. The actual dimensions of the (60mm x 70mm) are 2.75″ wide by 0.276-0.374″ high and 2.362" deep (69.85mm x 7-9.5mm x 60mm). The actual dimensions of the (54mm x 78mm) are 2.126″ wide by 0.197 or 0.315″ high and 3.091" deep (54mm x 5 or 8mm x 78.5mm). These drives have been used in small devices including as add-ons to game system.
The dimensions of a 1.8″ drive are specified in the SFF standard specifications SFF-8111 and SFF-8120 which were incorporated into the EIA-720 specification by the Electronic Industries Association (EIA).
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, fan controllers, RAID controllers, tool storage, and other uses. Some computers have a small system monitor LCD 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. Since computers have 12 volt rails on their motherboards, some computer hobbyist websites even sell addons for cigar lighter receptacles to power or recharge devices made to power from automobiles, though USB is already available for charging devices like cell phones and portable media players.
Drive bay-compatible computer case accessories that do not connect to the motherboard or power supply at all are also common, such as small storage drawers or even cup holders.