Back up to USB flash drive problems

I use a 32GB USB flash drive to back up my computer through Norton 360.  I am not able to back it up, as I get the following message:

There is a back up problem.  Make sure the destination drive or device is connected and try again.

 

My computer recognizes the drive as the Norton restore application and file folder are present.  I've even successfully saved other files to the drive as a test.  I thought maybe the drive didn't have enough memory as I have been doing this since I bought the drive in Sept. 2011, so I deleted the backed up files yesterday (which took hours).  Well, today, it still gives me the above message, so I am unable to back it up on the drive.

 

Please help!