We had this problem recently when restoring a company’s data after a crypto locker infection. We had the shadow copies which was the immediate fix and the main backup solution was Backup Exec 2014. On first inspection, I could not see the backup in backup exec. Anyway, for various reasons the shadow copies didn’t cut it and we needed to do a full backup from backup exec. We found that we connected the backup drives, ran inventory and then started the restore. The restore then failed, and the backup was gone from the backup drive!!
We found this link which was very useful!
It seems backup exec 2014 has a setting that deletes any backup on the attached drive if the retention period has passed which when left unconfigured is 1 day!
In the Job settings, under storage you’ll find this Data management option.
This needs to be un-checked and ideally the backup drive should be formatted before the next backup runs, not when the drive is attached!