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I suggest you to take help from this article http://www.wikihow.com/Repair-MS-SQL-Server-Backup-%28.Bak%29-Files-with-the-Help-of-SQL-Backup-Recovery-Software Which will help you restore bak file normal.
Hi marshall, the main reason for SQL backup file corruption is interval Server errors or virus attacks etc. These errors affect the entire data functioning of the backup file and eventually results in the inaccessibility of the data items. In this situation user can use SQL backup database recovery tool to recover corrupt backup file into healthy backup file by maintaining proper data integrity as usual.