Symantec System Recovery creates Full backups when server is rebooted

We use SSR 2011 to create images of two VM's.  We swap USB drives out weekly.  Just recently, I've observed that if we reboot the server or disconnect the USB drives and reconnected them, when the backup runs, it creates a full backup of the drives, not an incremental image as expected.

Just to be clear, I'm using the Full and Incremental nomenclature, but we are not doing file system backups, we are creating images.

We were using VMware ESX 4.1 and switched to Hyper-v servers.  I'm using a third party USB redirector to get the USB drives to mount on the VM's