So having been bit by the "upgrade fails" bug because \users is a junction point to the root of a different drive [I used U:\], I decided to try to move users back to C:\Users until after the upgrade and I'd then move it back.

Don't ask. I do it because if I have to reformat C:, I don't want to take \users with it in case of catastrophe.

Like now.

My C: drive (maps to G: in the system recovery environment) will let me:
list any KNOWN directory under the root, i.e. if I know exactly what directory it is, I can list its contents and access it but I cannot CD there.

It will not let me:
drive-cd to anything on it if I switch to a different drive in cmd or powershell
system restore

NOTHING. Not even in system recovery. All I get is "Access is denied". Weirdly enough, as mentioned, if I know where things are UNDER C:\, their permissions have not been munged, so everything there is fine. I just can't get to C:\ in the canonical way.

In the UI, when I try to even access security information, it tells me that I don't have permission. Attempting to use the advanced mode just hangs that window for about five minutes before telling me it can't get access to the drive. It also provides me no disk space information on this drive in the UI.

I would really rather not have to reinstall windows, as I don't have a system backup from which to recover at this point; I fear if I take one now, it will just reinstate the same problem on a restore.

Any help would be greatly appreciated.