A drive bay is the storage area where new hardware is added to an individual computer system. In other words, it is a system unit space reserved for the installation of any drive or other computer accessory.
Drive bays are used for mounting hard disks, floppy disks, tape, CD-ROMs and other drives.
Internal Drive Bays
These bays are entirely within the case and are not accessible from the outside. If a device does not require any access from the outside it is preferable to use an internal bay, and save the case’s external bays for drives that need them. In practical terms, this means that internal drive bays are usually used for hard disk drives, which do not require any access by the user.
External Drive Bays
External drive bays can be removed and accessed from outside of the computer tower. Despite the name, “external” drive bays are inside the computer’s tower case.
External drive bays are generally preferred as they free up the external buses for drives that need them.
You can of course mount a hard drive into an external drive bay. So in some ways, an internal drive bay is really an “internal only” bay. Some cases in fact do not have any internal drive bays; hard drives are mounted into external drive bays and solid faceplates left to hide the drive from the outside.
The two sizes of drive bays are 3.5-inch and 5.25-inch. The dimensions refer not to the actual size of the bays but rather the floppy disks that are typically inserted into them. The 5.25-inch floppy disks are increasingly rare. The 5.25 disks are at least 5.75 inches wide, 8.0 inches deep and 1.63 inches tall. The 3.5-inch disk has a width of 4 inches, a depth of 5.75 inches and a height of 1 inch. According to PC Guide, the depth numbers are the minimum required for drives to fit the bay.