How to Release a Converted VHDX?

I just converted a differencing VHD to a fixed VHD, but when I try to start the VM to which I attached it, I am told it is already in use by another process, effectively stopping the startup.

I looked at Task Manager, but I didn't see ConHost.exe running, so I am believing the Convert-VHD process has terminated.

I shut down the entire Hyper-V server but that didn't release it.

I tried moving the file using Windows Explorer, but it can't because it is in use by something, which validates the original situation.

Ideas?