dual install

I want to install Win7 on 2 H/Ds is there anything that I should be aware of before I start? One drive is PATA and the other is SATA I think I must select which drive to boot from in the BIOS' I realize I cannot run 2 instances of the same Win 7 at the same time and the end goal is to reduce...

I was just trying to keep a fresh up-to-date copy of the OS ready in case my working hd crashed. Is this possible? I'm currently experimenting with this as I've already installed it on the two different drives, but the second clean install won't validate? Is this a restriction by Microsoft, is...