At some point recently, my workstation became unable to map drive M: to a particular server share.

If I tried “net use m: \\server\share”, I’d end up with an error stating that “That device name is already in use” or better “System Error 85 has occurred”.

But, if I then did a “net use” to list the active mappings, drive M didn’t show up in the list.

I tried rebooting, both client and server, as well as a few minor registry tweaks (under the advice from one site that mentioned Windows will cache drive mappings in some cases), but nothing worked.

Finally, I just happened to open up the Disk Management screen from “Computer Management” (Right click on “Computer” and select “Manage”).

Notice the Drive M line and the “Removable” disk type. Ugh.

At some point, I must have inserted a new USB thumbdrive when I didn’t happen to have a connection (and hence a mapped M drive) to my server, and Windows just decided to map the thumbdrive to the empty M: slot.

But, when I got my server back up, even though the thumbdrive was long gone, that mapping was persisted and thus the slot was taken.

To fix it, I just right clicked on the highlighted line above, and selected “Change Drive Letter”. Remove the old M: mapping, and add a new W: mapping.

and the problem is solved.