Initializing Backup Media
When creating a backup on a tape backup device for the first time, Microsoft® SQL Server™ needs to initialize the backup media before the backup can be created. Initializing media causes a media header to be written and deletes any existing media header, effectively deleting the previous contents of the tape. When initialized, previous information on the tape cannot be retrieved.
Initializing disk media involves only the backup device file(s) specified by the backup operation. Other files on the disk are unaffected. When using backup devices for the first time, SQL Server automatically creates the file(s) needed by the backup device(s) for the backup operation. Reinitializing disk backup devices overwrites the contents of the files used by the backup devices and writes a new media header.
To initialize media for the first time when creating a backup