Here, again, folksies, is my intended system:

Two or more HDs are mounted in removable trays.
Periodically, I copy a clone file to one of several
partitions on each of them, keeping a running archive
of about 8 weekly backups of the entire primary HD.
These removable HDs are ATA/133 HDs and
they're both faster and cheaper than USB2 or
FireWire external HDs. Copies go faster, so I'm
more likely to backup often, and being cheaper, I
could afford to buy even another HD and removable
tray to mount it in, thus extending my archive's
temporal span.

A clone HD is inside my PC, containing a bootable
clone of the primary HD, for immediate use if the
primary HD should fail.

I haven't yet installed the removable HD components,
but I do have a clone HD constantly running in my
PC, and I remake the clone once a week, alternating
between 3 extra HDs that I have so as to have
time-series snapshots of my primary HD going back
through 3 backup points.

An objection repeatedly raised is that this is a "kludge".
So what? If it works, who cares if it isn't to "specs"?

Another objection is that the failure of the primary HD
is such an unlikely event that it doesn't deserve making
bootable clones. That presupposes that the time to
copy a clone file from an archive medium to a spare
HD that one manually substitutes for the failed HD is
irrelevant. Here's the scenario: I do stock trading in
the morning hours. At any time, a hard drive failure
could be catastrophic if I couldn't recover in a couple
minutes. At one time I had dialup, DSL and cable
internet service, and there were times that I was
thankful for the redundancy. Now I just have dialup
and cable (through 2 ISPs), but being prepared has
saved me a bundle a couple times, and it's worth the
effort. I also do software development during the rest
of the day for university courses I take. I like to have
separate one or two HDs for that purpose. I'd like
nothing better to be able to switch between HDs without
having to open the case, etc., and without having to run
all the HDs all the time. Thus - the desire to have power
switches to prevent unused HDs from being powered up
as well as to assist in isolation when booting up clone
HDs for the first time. A device toward that purpose
might be the Romtec Trios II, which would have the
advantage over manual switches by preventing a change
in power to a hard drive if the PC is running. The down-
side is that it takes up the space of 2 PCI cards. The
upside - it flouts the specs, of course! :-) And....
it's mindlessly simple.

*TimDaniels*