windows 10 apparently showing incorrect storage volume on external hds

#1
Seagate Slim Backup. Total Capacity: 931.

According to the properties in "this pc" there are only twenty gigs left on the drive but when I physically select everything in the drive it says the total space used is only 847 which means I should have closer to 90 gigs free rather then just 20. I defragment the drive, and empty the recycle bin and this frees up an additional 14 gigs but obviously there is close to 60 gigs still unaccounted for. I've also looked for hidden files and folders and found none. Also there are no other partitions on the drive.

Any thoughts on this would be appreciated. Thanks!
 

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