Although any hard drive can obviously be removed, the term removable hard disk drive refers to hard drives designed to be removed and reinstalled easily, without opening the case or disconnecting and reconnecting cables. There are two distinct types of removable hard disk drives:
Cartridge-based drives such as the Iomega Jaz and Castlewood ORB use a self-contained, sealed cartridge about the size of a thick 3.5-inch floppy disk. The cartridge contains only the disk itself. The head mechanism resides in the drive. You insert the disk into the drive much as you would a floppy disk. Inserting the disk causes a shutter on the disk to open, allowing the drive’s head mechanism to read and write the disk. The Iomega Peerless system instead uses a cartridge that is essentially the HDA (head-disk assembly) of a standard hard drive. Cartridge-based units are available in internal and external versions, using IDE, parallel port, SCSI, USB, PC Card, or FireWire interfaces.
Cartridge-based drives have always been niche products, but are now obsolete in practical terms. Their raison d'être, transferring moderately large data sets between systems, is now better served by a DVD writer or similar industry-standard writable optical drives. For most purposes, cartridge-based drives are now too small, slow, proprietary, and expensive. The Castlewood ORB is the only cartridge-based drive that remains in production.