Right, my bad.
If you unplug one drive to install the other OS, then you won't have dual boot, as you said. Sorry for the bad advice.
Does MAC have a Dual Boot scenario? I know Windows does, as I have a machine with Windows XP and 7 on it. Installed on one drive, differnet partitions. I don't see why 2 diffferent physical drives would be any different.
Don't expect a reply for Seagate, it's not a drive issue, it's a software issue.
Still, I can't help right now. I know nothing about Mac OS. I'll look around.
The site given by jefe323 is valid. It's the only way I would know how to dual boot Mac & Windows. Yes, Windows sets up the dual boot screen, where you choose which OS to run. But I don't see any problems with that.