Been futzing with this for hours now and it's driving me a little nuts. I have two user accounts on my Mac (High Sierra, MacBook Air, 128GB SSD), and the amount of free disk space in each shows differently both in the Finder and System Profiler. In my primary account, I see just over 40 GB free, with "13.33 GB purgeable". In my secondary account, I see 28 GB free (and a much smaller 266 MB purgeable). It seems like the secondary account just isn't seeing the space as free, which is a bit disturbing. Just out of curiousity, I tried copying a bunch of files over to the primary account, to see if it would purge enough stuff to make room for the copies. It didn't, and I was ultimately unable to copy them over.