Putting the system disk in a removable rack

don't know if this is the right place for this question, but it seemed like as good as place as any so here goes.

just for grins i got the idea of putting my c: drive with windows 7 into a removable rack. the idea was that i could put windows xp on a separate hard drive and swap between to two (machine powered down of course). the thought was that i'd have two independent computers in one box. i know i could dual boot, but like i said this was for grins.

it almost worked. with the c: drive moved to the rack, win 7 booted up and ran just fine. i shut down, swapped drives and did an install of xp on the new disk. although it was assigned drive letter d:, it loaded and ran fine. so i shut down to swap back.

after putting the windows 7 drive back into the machine, windows wouldn't load. i got an error message saying that the windows/system 32/hal.dll file was corrupt or missing. i checked the drive, it wasn't missing, so...corrupt. using the 7 dvd, i was able to do a startup repair and am back up and running. no harm, no foul and only a little lost time.

but i'm curious. is what i'm attempting possible? if so, how would i go about making sure that both os'es boot properly when the corresponding drive is in the rack?

many thanks,
brian