I've installed a fresh copy of SBS 2003, configured everything, then run a backup on to an External USB Hard Drive. Part of the way through the backup, the following event has appeared in Event Viewer > Application Errors:
"certsrv.exe (940) The backup has been stopped because it was halted by the client or the connection with the client failed."
However, SBS is reporting the backup was successful and that no errors were encountered. There's an event in the Application Errors database to back this up, as follows:
"Small Business Server Backup completed successfully. No further action is required."