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
Most such drives, including the Iomega Jaz and Castlewood ORB, use a self-contained, sealed cartridge about the size of a thick 3.5" 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 amounts to 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.
- Frame/carrier-based drives
These drives are actually just modified drive bays that allow a standard hard drive mounted in a carrier assembly to be inserted and removed easily. The frame resides permanently in an external drive bay, and is connected permanently to power and to the IDE interface or SCSI host adapter. The carrier assembly contains power and data cables, which remain permanently attached to the hard drive. The rear of the carrier assembly contains a custom connector that routes power and data signals from the frame. The connector that mates the carrier to the frame is designed for durability, and is typically rated for 2,000 to 50,000 insertions and removals.
These devices are simply physical modifications that allow easy removal and insertion, so the system sees the drive as just another hard disk drive because it is just another hard disk drive. Frame/carrier assemblies are available for any hard disk interface, from IDE to Ultra160 SCSI. More sophisticated units support such functions as hot-swapping, sparing, and RAID, if your host adapter, drivers, and operating system also support those functions.
External Hard Disk Drives
External hard drives are a related class of storage device, but do not qualify as true removable hard disk drives. They are similar to removable hard disk drives in that they allow large amounts of data to be moved between systems. They are dissimilar in that they do not use removable media.
External SCSI drives have been around for years, of course, but they have always been a niche product. External Plug-N-Play drives with USB or FireWire interfaces (or both) are seen more frequently nowadays, particularly with notebooks. In effect, these devices are simply standard IDE hard drives in an external enclosure with USB or FireWire interface circuitry.
The drives themselves perform as you would expect a modern IDE hard drive to perform. The problem is the interface. FireWire is fast enough to use as a hard disk interface, but few computers have a FireWire interface, and the cost of adding FireWire to both a PC and notebook makes this solution quite expensive. USB 1.1 is ubiquitous, but provides less than 1 MB/s throughput, which is too slow for reasonable hard drive performance. As USB 2.0 which matches FireWire performance replaces USB 1.1, these devices will become more practical.