The error arises when filemark in the backup device could not be read. There are many reasons why you may encounter a filemark error. Some of the reasons include the following :

A media failure may occur on the device where the backup is located.

A write failure may occur during the creation of the backup.

To allow SQL Server to perform new backups to the backup device, you must manually delete or erase the device by using the following command:

BACKUP DATABASE mydatabase TO DISK='C:\MyDatabase.bak' with FORMAT

For more information about error solution, please check this link : http://support.microsoft.com/kb/290787