As
meebers commented, you can, as I have, 2 HDs with the same build of Windows 7 & key, using the bios to boot into which ever one is required; should 'C' fail, then I've got immediate access to another on 'D' which when booted, becomes 'C'.
I never use multiple boot options; on one occasion, heaven knows how it happened, the OS on the 'C' drive required files from the 'D' to function so if 'D' was removed, then 'C' wouldn't boot.
Wasn't aware that the context menu (pointed out by
lehnerus2000) had an option to overcome
signature collisions with another disk ; as mentioned, I resulted to Diskpart to change the ID.
If you had another copy [with its on key ]of windows on the second hard drive you likely would not have gotten the error
But then surely this wouldn't be a cloned copy of the OS, unless the key is changed at a subsequent stage.
As I only want to run one OS at a time, then there's no problem in using the same key,