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Publisher8.1 Uses for Removable and External Hard Disk Drives
Although most systems do not need a
removable or external hard drive, such drives have the following
uses:
Installing a larger hard drive in a desktop system is trivial, but on
a proprietary laptop system it may be impossible or extremely
expensive to upgrade the hard drive. If you're faced
with this situation, using a removable or external hard drive may
extend the usable life of the laptop. If the laptop has an available
PC Card slot, install a USB 2.0 PC Card adapter so that you can take
advantage of the much higher throughput of USB 2.0 external drives.
If you need to transport large amounts of data to remote sites, a
removable hard drive may be the only practical option.
Cartridge-based removable hard disk drives store up to 20 GB, and
frame/carrier-based removable hard disk drives are limited only by
the capacity of the largest standard hard drives available. For
example, one of our readers works for a company that produces digital
special effects for movies, always on very short deadlines. The time
needed to back up 100 GB of image data to tapeas well as the
cost and complexity of installing the required $35,000 tape drives at
each endmakes tape impractical. Instead, they install a
frame-based removable hard disk drive system at each end, copy the
huge datafiles to high-performance SCSI hard drives, and FedEx the
hard drives to the movie production company. When the disk arrives,
the production company staff simply plugs it into the frame and
copies the data from the removable hard drive to the server. Now that
USB 2.0 external hard drives with capacities of 250 GB or more are
available at reasonable prices, we expect them to be used for this
purpose.
Software development and similar work that requires using multiple
operating systems always presents a problem. You could configure one
PC to multiboot different operating systems, but that is seldom
entirely satisfactory. You could install a dedicated computer for
each OS, but doing that is expensive, generates a lot of noise and
wastes heat, and means you're soon covered up in
computers. Or you could use one computer with a frame/carrier-based
removable hard drive. Installing each OS on its own hard drive means
you can simply insert the carrier with the appropriate OS, restart
the system, and have the equivalent of a dedicated PC running that
OS.That's why we use a frame/carrier unit on our main
test-bed system. We know one person who does the same on his home
computer. He, his wife, and his children each have their own hard
drives mounted in carriers. He uses Linux, his wife uses Windows
2000, and the kids use Windows 98. When someone wants to use the
computer, he inserts his own hard drive and restarts the system to
boot into his own environment, with no worries about conflicts or
accidentally damaging someone else's data or
configuration.
Although it is a specialized application, some scientists, market
researchers, and others need to manipulate extremely large data
setssometimes in the 100 GB range. Although it may be possible
to build a PC with sufficient disk space to store all the data sets
on the main hard drive, it's often cheaper and more
efficient to allow those data sets to be swapped in and out as
needed. If there are many such data sets, using a frame/carrier
removable hard drive may be the best option.
Although one or another tape technology is usually the best choice
for routine backup, there are times when tape is simply not
practical, either because of the time required to back up or the time
required to restore. For example, assume that you have a 150 GB
database stored on a mirrored set of Seagate Barracuda 180 Ultra320
hard drives. Your backup window is very short, and no tape drive is
large enough and fast enough to back up that entire 150 GB in the
time available. Furthermore, even if tape were workable for backup,
you can't afford to have the database offline for
the time it would take to restore it.If the frame/carrier unit, the OS, and the host adapter all support
hot-swapping and RAID, you can remove and insert drives as needed
without downing the server. In that situation, you might install
three frame/carrier-based removable hard drives, each with a Seagate
Barracuda 180 hard drive. Two of those would be mirrored/duplexed to
provide primary storage. The third would be used as a destination for
an image of the working data set. The 160 MB/s transfer rate of
Ultra160 SCSI theoretically allows you to transfer the entire 150 GB
database to the target drive in about 32 minuteswith half the
160 MB/s throughput used for reads and half for writesalthough
in practice it might take twice that long. Still, in comparison to
even a very fast tape drive, the disk-to-disk transfer takes no time
at all. Also, that backup hard drive can be replicated offline on
another system for redundancy. With a recent backup stored on a hard
drive, if you have a catastrophic failure, you can simply insert the
backup hard drive and run directly from it, without spending the time
needed to recover from tape.
If you work with extremely sensitive data, using a removable hard
drive allows you to secure that data by taking it with you or by
storing it in a vault. When we first encountered a removable hard
disk drive many years ago, this was the reason that drive was being
used. The corporation ran two payroll systems, one for executives and
the other for everyone else, and was paranoid that an employee would
find out just how much more the executives were paid than everyone
else. The executive payroll system used a removable hard drive, which
was secured in the vault immediately after the executive payroll was
run. And their fears were perhaps justified. If it's
any indication, when we had to troubleshoot a problem with the
payroll software on the executive payroll computer, it turned out
that the problem was that the field for monthly salary could not
exceed $99,999!
