Acronis True Image Home installs a Windows service called "Acronis Nonstop Backup Service", used for a feature of the same name. When this service is started, it seems to interfere with USB drive unmounting in Parallels 6. Specifically, if one uses the "safely remove hardware" feature in Windows (Windows 7 32-bit) in this case, the drive gets stuck in some sort of limbo. It still appears in Windows, but can no longer be accessed. It will not be accessible again until the system is restarted. When restarting, Windows 7 will hang forever. The solution to this seems to be to disable the nonstop backup service (which fortunately does not affect other features of True Image aside from nonstop backup). I'm not sure who is more at fault here, but True Image works just fine in VMware Fusion 3.1, so I hope someone will look into this. I wasted a lot of time tracking this down.