Management of file extent mapping to hasten mirror breaking in file level mirrored backups

An enterprise storage system for performing a file level backup operation on data using a mirror disk and a primary disk includes a split mechanism to split the mirror disk from the primary disk before backing data files up from said mirror disk to said sequential storage media and an extent mapping mechanism to perform extent mapping on files to be backed up in a given backup session before the split mechanism splits the mirror for the same given backup session, and an extent map check and update mechanism to check, after the split mechanism splits the mirror for the same given backup session, whether any files have been updated in a manner so as to require additional or revised extent map information.