I recently installed a second 8Tb SSD in my Mac Pro:
It appeared to work fine out of the box as configured. But somehow the drive is nearly full now even though the Finder shows it should be about half full.
What has used up 3Tb of space? First Aid on the Mac says it is working properly.
If you’re comfortable with the command line, du -hs ls -A at the top level directory of the disk and then descend the directory tree as required. Nearly 100% of the time, the directory that’s holding all of the unexpected data will stand out like a sore thumb.
Download omnidisk sweeper (free) and you can see a nice representation. It’s not time machine back up is it?
Yes, it is being used for a Time Machine backup. The Time Machine folders show 3.2 Tb used (fairly typical as about 1.8 Tb is used on the drive it is backing up). Does that explain it somehow?
To add to the puzzle… both Omnidisk and CleanMyMac show that the SSD with 7.68 TB capacity has more Used space than actually exists on the drive - how can that be?
