After weekend updates, several servers showed a random new mapped drive – but only in file explorer. It did not show up in Storage Management. Looking at it, some update had decided the EFI partition needed a drive letter….
This was on Server 2012R2, by the way – here’s the fix:
Administrative command prompt;
list volume (find the one about 100 mb)
select volume #
And that’s it – ghost volume gone, EFI volume still where it’s supposed to be. Here’s the link I found that explained it to me….