I wouldn't be worried about the backup right about how, I'd be worried about the database itself. That error is on the mdf file.
Please run this and post the full output.
DBCC CheckDB(<Database name>) WITH No_INFOMSGS, ALL_ERRORMSGS
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability