My VMs are configured to save state, and restart if previously running when the host server is rebooted. All of the VMs start up upon reboot... however, some of them start from a cold boot, and not from a save state resume. Why is this occurring? I've searched for any relevant hotfixes, but I couldn't find any. The host is Server 2008 SP2 x64. I can't find a correlation between which guest VMs fail to properly resume, sometimes they do and sometimes they don't.
Your IO subsystem is too slow.
Windows is not waiting forever for all the VM's to shut down. Once the timeout is over, it restarts.
The only things helping are:
Running 4 Hyper-V servers here and - well ;) Same problem. Got better since my main one was upgraded to 8 discs + 2 SSD for the VHD files ;)
check the event logs of the VM guests that started from a cold boot to see if they "unexpectedly rebooted" or crashed.
also, double-check the settings for your VM's. there are three possible automatic stop options:
you might be confused by the shortened version of the last 2, which are "Save" and "Shut down".