You aren't giving enough information for us to help.
How are you doing your backups (maintenance plan, scripted job, or vendor product)?
Where are you backing up the database(s) to (drive on the server, network drive, tape, CD, something else)?
Are you running anti-virus?
What is the true size of the database? How much space is used and how much is free/unused space? Has that changed recently?
Are you doing anything else at the same time?
Have you checked the error logs (SQL Server Error logs/Windows Event Viewer logs) for any possible related messages?
-SQLBill
The following is part of my signature block and is only intended to be informational.
Posting advice: FAQ481-4875