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
"Ineptocracy- a system of government where the least capable to lead are elected by the least capable of producing, and where the members of society least likely to sustain themselves or succeed, are rewarded with goods and services paid for by the confiscated wealth of a diminishing number of producers." - Merritt Man, 2011