Second the OS is licensed per machine (most licenses) so you are free to install it on as many drives as you wish on that PC.
Should be fine.
I have fitted a Mitron Duplus Hard Drive Switch to my desk top computer. I have installed two new hard drives. I have installed a Windows 7 Professional retail OS on one of the drives.
My question is - could I legally install the same OS on my second hard drive. As I understand, it is the chip in the motherboard that recognises the OS, so perhaps it will see it as the same OS.
As the OS is full retail, I don't think I will be doing anything illegal, but thought I would check with you good people before I proceed.