Yes sort of.
When I perform a complete system backup of only the fixed hard disks, where the volumes are:
C, D ,E and F - fixed hard disk
Y and Z - external USB hard disks
the VSS creates its files on Y and Z because they have the most free capacity, but these are the drives I backup to so I sometimes get "ftdisk" and "volsnap" errors due to excessive traffic.
I think VSS thinks the USB external drives are fixed so it thinks they can be used. I need to exclude them somehow.