Does Hyper-V support GPT disks on the host

I suspect the disk is just offline in the guest. Did you go do Disk Manager in the VM and bring it on line?

I have to say, though, it's not worth bothering with. I'll bet that if you try to measure the performance overhead of putting fixed VHD on your disk and assigning that to the VM, you won't be able to tell the difference. And if you use the VHD, you can migrate the VM more easily. You can back it up. Etc.

Your host ran out of disk space (or thought it did, can happen with flaky clusters or SANs)