Important lesson: Back up your data files before you revert to an earlier version of Windows. If you lose a file while going from Windows 7 to Windows 10, you can usually find it from inside Win10 in the hidden Windows.old folder. But when you go back from Win10 to Win7, there is no Windows.old folder.
Impediments to rollbacks
Microsoft promises that you can upgrade to Windows 10, then roll back, if you perform the rollback within 30 days. While that’s true to a first approximation, the details are a shade more complex.
When you perform an in-place upgrade from Windows 7 (or 8.1) to Windows 10, the installer creates three hidden folders:
Those folders can be very large. Upgrading from a clean Windows 7 machine with Office 2010 installed, C:\Windows.old runs 21GB.
Deleting the hidden C:\Windows.old folder, either of the other two folders, or any of their contents, will trigger a “We’re sorry, but you can’t go back” message (screenshot). Those are the folders that hold all of your old system, including programs and data. Generally, it’s difficult to delete the folders manually, but if you run Disk Cleanup in Windows 10, opt to Clean up System files, and check the box marked Previous Windows installation(s), your Windows.old folder disappears and can’t be retrieved.
(Older posts suggest that running the Windows Media Creation tool will delete the $Windows.~BT folder. That may have been true six months ago, but it looks like Microsoft fixed the problem.)
Although it isn’t well documented, apparently the Win10 upgrade installer sets a Scheduled Task to delete those files -- they take up a lot of room, and understandably, Microsoft wants to give that room back to you. I couldn’t find any associated setting in Task Scheduler, nor could I find any documentation about the task, so the removal of those files after 30 days may be more complicated than most assume. Others have found that moving (or renaming) those files, then moving them back after the 30 days has expired, does not reload the rollback mechanism. If you think you can be tricky and hide the files, returning them when you want them, I’ve found no indication that’s possible.
You can, however, roll back from Windows 10 to Windows 7, then roll forward again. By rerunning the downgrade/upgrade cycle within the 30-day window, you’re good for another 30 days. I’ve rolled back and forth four different times on the same machine, with no noticeable problems.
There are other situations where either Windows.old never gets generated, or it is stripped of all of your programs and data. That’s what happens with a clean install.
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