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
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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.
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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.
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