Removable HD as backup in XP?

I just went through this because I thought my old 80G was
dying (high pitched whine that varied irregularly in volume
developed suddenly).

Depends on how lucky (and knowledgeable) you are. I did
just what you are talking about. Couldn't get my system to
clone the old 80G on to a new 120G. It finally did, but I
still don't know for sure what the problem was. Second, I
cloned the old 80G onto a new 80G, but it took several
tries. I had originally tried with the Western Digital's
Data LifeGuard Tools, which copied the data ok, but would
not make a bootable drive. I think I managed to mistakenly
put EZ boot on the original 80G, and that may have caused
most if not all of my problems with cloning to the 120G and
definitely caused a problem with cloning to the new 80G.
Other possible problems on the original 80G may have been
fixed through defragmentation and various checks.

Both the new 80G and the 120G, will boot fine now. However
a couple of extra lines were put into the boot.ini on the
120G (probably by some stupid thing I did). The system then
thought that there were two operating systems and ask if I
wanted to boot with XP. After removing the 2 extra lines,
it no longer asks that question.

These drives were swapped in and out, in various
combinations, many times. Other than the problems stated
above there have been no problems with XP. I have XP Pro,
and used Partition Magic and Drive Image in my attempts at
cloning. The final clones were made with Drive Image.

I would say that if your system is in good shape and you
have a clue what you are doing, you probably won't have any
problem. BTW, Gateway told me I wouldn't be able to clone
the drive and would have to reinstall the software.

Cheers.