When you install OS's on separate HD's in order to (wisely) boot via BIOS, you must unplug the other HD during installation so it won't interfere.
Then set one OS HD as first to boot in BIOS setup (after DVD drive) and boot the other OS HD's using the one-time BIOS Boot Menu key:
Asus - F8
HP/Compaq - Esc
Sony - F2
Acer - F2
Gateway - F10
eMachnes - F10
Toshiba - F12
Dell - F12
IBM/Lenovo - the blue Thinkvantage button
This keeps the HD's independent to come and go as you please, without GRUB corrupting Win7, and without interlocking as with a Windows-managed Dual Boot.