Hi Lone Star, Changing of the hard drive on the systems will not prompt you for activation but after swapping the hard drive on a different system if the operating system is validated activation would be prompted as the machines physical address would have changed. 
I'm new to Windows 7.    I frequently swap out (rotate) the hard drive on my system--say every 6 weeks or so.  I use the same set of 2-3 drives and restore my system from a drive image each time when I swap them.   Is changing the drive this often going to cause any problems with Windows 7 activation?   Thanks.
Do frequent hard drive swaps cause Windows 7 activation problems?