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.
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 Ultra320 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 and Play drives with USB or FireWire interfaces (or both) are becoming increasingly popular, particularly with notebooks. In effect, these devices are simply standard IDE hard drives in an external enclosure with a USB or FireWire interface.
The drives perform as you would expect a modern IDE hard drive to perform. In the past, the problem was the interface. FireWire was fast enough to use as a hard disk interface, but few computers had FireWire ports and the cost of adding FireWire to both a PC and notebook made this solution quite expensive. USB 1.1 was ubiquitous but too slow for reasonable hard drive performance. In 2002 systems began shipping with USB 2.0 interfaces, which are more than fast enough to support any current hard drive.
Pioneered by Maxtor with its Personal Storage 3000-series drives, external USB 2.0 hard drives proliferated as USB 2.0 became common. Competing models are available from Maxtor, Western Digital, Iomega, CMS, QPS, and others with capacities as high as 250 GB or more. Although these drives can be used just like any other hard drive, they are marketed as backup/archive solutions. Makers generally bundle software such as Dantz Retrospect that allows backing up your internal hard drive to the external drive with just the push of a button. As the drive fills up, it's an easy matter to delete old backup data to make room for new. We have serious concerns about using an external hard drive as your only backup solution, but it's undeniable that these drives make backing up fast and easy.