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.
Corky-G - Tucson, AZ
"In my many years I have come to a conclusion that one useless man is a shame, two is a law firm and three or more is a congress." John Adams