Most of the time, this is caused by the drive being full. You can even see information from Microsoft on this. There is a race condition in 2005 that can generate this error, but it's got a hotfix available. However, you're on 2008R2, so things are different. You may also see this error when using some third party backup tools.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning