Cannot backup database, error saying not enough storage when there is plenty

I need to backup a SQL database to a flash drive that I have inserted into the computer with MS SQL on it. The database is ~80GB and the flash drive has ~240GB free. When I point the backup to the flash drive, I get an error telling me there is not enough space for the backup.

System.Data.SqlClient.SqlError: A nonrecoverable I/O error occurred on file "D:\bdx.bak:" 112(There is not enough space on the disk.). (Microsoft.SqlServer.Smo)

I have tried running a query to perform the backup with the same results.  I have also tried using a backup device pointing to the flash drive and still get the same error.

Thoughts?  Thanks in advance for any help.