Remote Volume GUIDs driving you crazy?
I was maybe half an hour away from giving up on my Hyper-V Server Core box and doing a "Full" install. The solution, once you know it, is simple... the problem was there seemed no way to reattach a previously created virtual machine. I hadn't done an export, so "import" wasn't an option. Usually, what you do in this situation is you create a new virtual machine and attach the old VHD file. Since I'm planning to use Failover Clustering, the VHD files reside on a iSCSI SAN and instead of drive letters, I'm using Volume GUIDs.
Almost all has to be done remotely, because the Hyper-V box is a Core machine.
First, how do I get the GUID of the volume where my VHD file is stored? On the Server Core box, open a command line and type "mountvol". You will see something like this:

It's obviously neither "C:" nor "D:"... so which is it? The most intuitive thing would be to copy/paste the path(s) and prepend them with "dir ", then go through them in order. However:

No such luck. Try the same, but add an additional backslash at the end this time:

There you go. Now the process of remotely attaching the VHD file in Hyper-V Manager is straightforward:

What still won't work is using the remote tools to browse a path that contains a Volume GUID:

If you want to remotely browse remote volumes, you should clear the path and then click browse:

Once the disk has been attached, you can inspect it, even though it uses a Volume GUID instead of a drive letter:

In conclusion, one more "\" can sometimes be all the difference in the world...