re: I don't know.
Rarely even works for internal drives. The hardware drivers for computer 1 will usually be incompatible for computer 2.
If you can select boot drives from MB bios at bios init, you can run different Os's on different drives W/O relying on any partitioning. You can run a full-security zone on one with your financial data, and run a bit more 'open' on the other. There are some other advantages over partition as well. I did something simmilar to this once with a work machine, don't remember why. I think I wanted to run a test server on Windows and keep linux for everything else.
DL