1.3TB of 'Other Files' showing up on My Passport ! Why?

My Passport 2TB drive is almost full. The drive was installed a couple of years ago as backup for my PC. The drive is permanently plugged into the PC. My PC has a total of only about 250Gb used disk space.
Having looked into this, I see that on the WD Passport, I have only 371Gb of ‘Data File Backup’ (as I would have expected) and 1.34TB of ‘Other Files’ leaving only 650Mb of free space.
What are these ‘Other Files’??
I need to get rid of them but don’t know how to do this.
Any help appreciated…

Hi @malc2001,

Please share screen shot of disk management to assist in best possible ways.