System Information reporting 1.51 TB in "system files"

[glawrie]

I'm trying to work out how to reduce the massive 1.51TB allocation to "system files" on my MacPro (5,1).
See image attached.
The computer is running Mojave (macos 10.14.6).

I have had a look at the drive using OmniDiskSweeper, and it reports just 492GB of files on the disk.
See image attached.

Likewise, Disk Inventory X finds just 492GB of files.

So there is about 1TB of stuff unaccounted for.

Anyone got any ideas about what might be using the space (and more usefully how I can get rid of some or all of it)?

Thanks in advance for any help.

 

[glawrie]

So there is about 1TB of stuff unaccounted for.

So progress - I created a backup of the boot disk using Carbon Copy Cloner set to copy 'all files' rather than 'the whole disk' and it duly cloned the boot drive to a spare and taking up just 492GB of space (i.e. leaving the 1TB behind). The cloned drive booted up OK and does not appear to have lost any data - so next step is to simply clone the newly created drive back to the actual boot drive and the space (hopefully) will be freed up again.

From what I can glean, the 1TB of junk files is probably APFS Snapshots created by time machine. In theory (according to Apple) these snapshots should not interfere with normal operation of the drive, as macos should remove them as it needs more space on the drive, but there are various reports of this not working out in practice.

A CLI command to disable their creation was removed for OS X 10.14, so under Mojave at least you cannot prevent time machine from littering the drive with these snapshots.

It is possible to remove the snapshots using obscure CLI type commands, and (although I have not tried it) toggling Time Machine off / on again might also clear them.

HTH