Look up booting from seperate HDD's and boot.ini files. You can have 2 OS's on one HDD and boot from etheir or, but from 2 seperate HDD's ... hummm need to select which disk to boot from , maybe a BIOS setting ?
"Whats bred in the bone, cannot be bred out" -
Robertson Davies
What could have happened is in your boot.ini file look to see if there are 2 instances of windows in it. If so remove one of them, then reboot and it should work.