Clean System Data on Mac
I store almost all of my files on external drives, yet my Mac has 487.35 GB used out of 494.38 total. 295.76 GB of it is System Data. I tried using the Terminal to find and clear TimeMachine backups, but Terminal shows me no TM backups whatsoever. I tried CleanMy Mac X and it cleaned about 3 BG of data from my drive, not even touching the System Data clog.
Only 109 GB are applications, the OS takes up less than 28 GB, less than 56 GB are documents and "Music Creation."
What other options do I have? Do I need to take this computer to a Genius Bar?
iMac 27″, macOS 10.13