Yupp, the above combination is really not great. Scenario: Hyper-V machine, several virtual machines, some with snapshots, various very long-running installations and a lot of work in these machines. New machines are created based on existing images, which are each generalized with sysprep and prepared for first use and then configured.

Enter the system administrator: a new virtual machine created, sysprep running, unfortunately not in the virtual machine, but on the Hyper-V server. It was then gone. First panic attack.

Colleague has revived the (of course remote) Hyper-V server and put it back into the domain, I get on. All configurations still there, all virtual machines still there. Not a single one of them works. Second panic attack.

Trying to edit virtual machines, no go - the configurations are not accessible, Hyper-V thinks they are all on drive C:. Checked, oh, the drives I: and J: (where the machines were before) are no longer there, have different letters. Ok, letters reversed and Hyper-V restarted. None of the machines run, they still think they are on C:. Third panic attack, as I realize that no configuration changes can be made.

Well, even in the configurations and the registry there is nothing about this mysterious C: - where does it come from? After a long search, found, for each virtual machine and for each snapshot Hyper-V places symbolic links under NTFS. These are located under %systemdrive%:\ProgramData\Microsoft\Windows\Hyper-V in the subdirectories "Virtual Machines" and "Snapshots" and point to the real target files. And in a magical way, all of these pointed to C: - apparently "corrected" NTFS at startup defective symbolic links that point to non-existent drives. Great.

So the links were recreated (first only an unimportant server, so I can see if it works). Of course it doesn't work, because Hyper-V ignores the nice new symbolic link. Permissions are wrong. Icacls can fix that - "NT VIRTUAL MACHINE\" is the syntax for the pseudo-user that needs to be assigned. But it doesn't work. Fourth panic attack.

Found, while swearing, that a Frenchman also had problems with this - Microsoft in its great wisdom has localized the names. Under the German version, therefore "NT VIRTUELLER COMPUTER\". Great. Really great. It only cost me 2 hours of my life.