If anyone else is using Parallels 12 on a Mac this may be of use to file away somewhere.
I run my development FM 15 Server in an OS X El Capitan Parallels environment. Host machine is OS X Server, VM is OS X. FileMaker Server runs on the VM. Generally works well for me.
Upgraded to Parallels 12 when it came out a few weeks ago. Had to replace my backup drive as it wasn't big enough. The new drive is 4 tb, USB 3.0. The Host computer would see the drive but when I tried to attach it to the VM it would vanish - neither the VM or Host would see it, nor would Disk Utility on either side. Only way to get it back was to reboot the Host machine.
Parallels tech support had no idea on what the problem was and told me that the drive (G-Technology) was probably the culprit. According to Parallels, if it doesn't say "Parallels compatible" on the outside of the box they can't officially support it. When I asked which drives were marked as "compatible" I didn't get an answer. Buggers. Tried a few brands of drives (LaCie, G-Technology and a generic) with similar results. Only thing in common was USB 3.0 and they were large drives (4 or 5 tb).
Turns out, if you disable USB 3.0 support in Parallels 12 all works fine. Tech support didn't know that. A Parallels forum member suggested it.
The drives mounted fine with a Mac OS X Host and Windows 7 Pro VM. The combination of a Mac Host and Mac VM seemed to be the issue.
Maybe its time to try VMWare.