Dear Sir:
I would suggest that you delete all of the configured B2D devices that point to the USB disk, disconnect any of thise disks from the server, restart the BE servces, and connect one of them. You should be able to configure that single USB disk as a regular Disk Storage device.
Repeat the operation for the remaining disks, and test swapping rhem and using them to make sure BE works correctly with all the disks,
Please check the Administrator´s Guide chapter on disk storage to make sure you understand how BE handles these devices in the new version, specially in what regards to space reclamation.
http://www.symantec.com/business/support/index?page=content&id=DOC5211
Additionally, can you please indicate tha make and model of the USB disks that you are using.
Regards,