Dont know about driver signing, I'll have to check but I cant find any reference in any of the group policies regarding USB Mass Storage. I have had a look at the gpo information about disabling support for USB Mass Storage with a gpo.. you have to install/create another administrative template, and there isnt an administrative template there that has that in it. Also, none of the client computers have a problem with USB Mass Storage so if there was a gpo preventing it I would expect it to not work on the client computers as well as the server. Cant see any server specific gpo's.
The server is a SBS 2003 server.