I use a hardware solution for that - relatively foolproof. One machine has two separate boot drives, and control is by means of a power switch I installed on the front panel. IOW, only the drive I want is powered.
On a newer machine I use a EZ-Swap 4 SATA HDD mobile rack. I simply insert the drive I want and boot. I have a choice of 6 boot drives with that. 2 Win 8, 2 Win 7 and 2 XP Pro.
CorkyG - Tucson, AZ
... Lack of transparency is a huge political advantage. And basically, call it the stupidity of the American voter or whatever, but basically that was really really critical for the thing to pass...... Dr. J. Gruber, ACA Architectural Consultant