I saw a post today on MS Technet , from an MS employee,answering another piece of the puzzle in regards to Win 7 upgrade media.
Quote: Michael - Support Engineer
If you boot from the upgrade DVD, Windows 7 setup will do a compliance check prior to the format. The results of the compliance check are stored in RAM at this point. After the format is complete the 'comliance information' is written back to the hard drive before the reboot happens. This is why it’s critical the format is done with the Windows 7 DVD.
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