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
SSR 2011 does not like swapping of USB drives as a backup destination, especially for base+inc jobs. It is recommended to setup a separate job for each drive, or use the offsite feature by backing up to a fixed location and allowing offsite to sync to the 2 rotating drives.
A new base image is usually created in place of an incremental when SSR believes the incremental chain has been broken from the base, and thus starts a new base image to prevent data loss.