Disk Space Warning

Hi Forum

We have a 1.7 TB Disk server.

According to Plesk Server Health we are using 60.2% (1.07 TB of 1.78 TB)

Plesk Tools Summary reports shows Disk usage 106 GB. (That kind of matches my plesk back up size which is about 29GB allowing for compression?)

I'm confused by what is using the remaining 964Gigs.

Any suggestions where I should look?

Thanks