The File Allocation Table (FAT), which tracks what files are where on the disk, how much space they take up, etc is corrupt.
Fortunately, there is a backup copy of the FAT on the disk, which Norton is offering to restore as the main FAT. Let it.
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Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.