I ended up purchasing two new hard drives for my system, and quite frankly ended up only using one of my total 3 for wow installations. After finding out about the junction/symbolic link method (NTFS junction points) I see no reason to have installations on multiple drives, save maybe having a single hard drive for every single instance of wow you run. Also, don't run any of the wows at all on your OS hard drive.

Basically, IMO, make one installation and several copies using the above linked method on your secondary hard drive (without the OS on it) and just roll with that. I've had no issues.