Found a good little article on clearing up “purgable space” on OSX Drive.
One drives which will unmount there is this useful command: diskutil secureErase freespace 0 /
Supposedly after running this command and restarting the computer, the purgable space will have been purged. I ended up not being registered in Disc Doctor right away with me, but was the next day.
Otherwise theres [this post(https://bambielli.com/til/2018-01-07-purgeable-space/) based on [Stack Overflow post](https://apple.stackexchange.com/questions/254676/how-do-i-clear-the-purgeable-area-on-my-disk)