We’ve been asked a few questions about pass-through disks.
At the moment, our Hyper-V backup solution only works when the Guest HDDs are VHD files, not physical volumes. (A pass-through disk is where the disk is connected to the host, marked as “offline” on the host, and passed through to the Guest where it is “online”.)
This is a limitation in the Microsoft Hyper-V VSS Writer, and also our Hyper-V Granular Restore Console only works on VHD guest HDDs as well.
The good news is that there’s no compelling reason to use pass-through disks anyway, so this limitation is almost a moot point. There is very little performance difference between a Fixed size VHD virtual disk and a physical volume, as explained in the MS blog: Working around the pass through limitations of the hyper-v vss writer.aspx
Happy Hyper-V Backups!