Hiya,
No error message at all, either in output from the proc, SQL Server error log, or Server event logs. Scary! Another process grabbing the device - as was the case with McAfee - seems the most likely but so far I can't spot it. I've double checked the folder is excluded from antivirus scans, checked backup logs, checked SQL Agent and Task Scheduler for simultaneous jobs - but so far nothing.
Cheers
Charles