While working on demo images that I deploy on VPC (Virtual PC 2004-2007) I noticed an interesting thing that makes a perfect sense though.
I use the newest feature of Virtual Server 2005 R2 (and I love the feature) - Virtual Disk Mounting. That basically requires Microsoft Virtual Server SCSI disk device driver.
What I noticed is that if I leave the VHD (virtual disk image) mounted in the OS on my development machine and I try to boot that image in VPC - I catch the UNMOUNTABLE_BOOT_DISK BSOD right away.
This makes sense since the volume is not available for writes from within the VPC as the host OS locks the VHD image with the new disk device driver in the chain.
The tip is simple - don't do that! Don't mount the VHD and try to boot from it in VPC.