Qemu snapshots to backup Windows

Qemu's snapshot feature is the best backup solution for Windows that I have found.  Although Windows System Restore is similar, it operates on a completely different layer.  My Windows VM has a 40GB C drive that is on an SSD and a 200GB D drive on regular spindle disk.  I rebooted my VM and something failed during the boot up process, which forced me to reboot the Dom0.  When the Windows VM came back up, my D drive was missing!  I opened the disk partitioning tool and it told me the disk was fine but there were no partitions.  If I relied on System Restore, how would I restore this disk?  If System Restore is part of the file system, and the entire partition disappears, what do you do?  Instead, I shut down the VM and applied the last snapshot that I created.  Funny enough, the last snapshot was created right before the reboot that hosed the virtual disk.  I have a cron job that creates a weekly snapshot of the disk.