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Chapter 8. Removable Hard Disk Drives

Chapter 8. Removable Hard Disk Drives

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

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.

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 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.


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