Playing Steam games on different versions of Windows from same location?

Solution 1:

Yes, it's fine as far as the game files are concerned; I've done this with an external drive on two different PCs, and after upgrading PCs.

A problem does arise however when game saves are stored on the Windows partition; some games do this, some games don't. Steam Cloud games should be fine either way, but certain games might be looking in (Active Windows Drive)\Users\My Documents or something for saves, so you'll end up with 2 save files, one on each OS. If this is a problem you might want to look at making junctions so both game saves are accessed at the same location. Alternately, if the My Documents folder in particular is the problem, I think Windows lets you move the location of that without a junction, too.