Two HDs, each with OS

[flatline]

You have to set one as master drive and one as slave drive. Its done with jumper settings on the back of the hard drive. Google it to find jumper settings for your particular drive.

[pokergrader]

No need to mess with the drive jumpers.

Just go into BIOS and select which one you want to be the boot drive. The master/slave designation for an IDE controller has nothing to do with which one can be accessed first.