Hang on.
You're backing the transaction log up every 15 min, writing to the same file and specifying overwrite? Is that correct?
Why full backups ever 2 hours? That's awfully frequent, especially with a diff every hour.
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