Lost space on My Book

Hello all. I am new to the forum. I have a 1 TB My Book. Over the last few weeks it suddenly filled to near capacity without my adding any large files. It was nearly half empty and now has only a few hundred megs of space. I have show hidden files selected and do not have a paging file on the drive. If I highlight all folders and files and check properties it only shows 262 gig used. I can do the same using TreeSize and can only find 262 gig of used space of the 1 TB. The drive is formatted with NTFS.  I have run virus scans on the drive as well as Malware and cannot find a reason for the space to vanish so quickly or what is using the space. Has anyone seen something similar? Thank you.

Try using a program to test for bad sectors and corruption

defragment and use chkdsk to test

you can also use HD Sentinel 

Backup all of your files and format the My Book.