Thanks to an Apple support forum, I found out you can do some terminal tricks to fix this. The problem relates to on-disk Time Machine backups, and how macOS stores them. macOS is supposed to deal with this automatically, but I like to speed up the process so I can have a better understanding of what is using my disc space.
- In Spotlight, launch Terminal.app.
- Copy and paste this command into Terminal:
sudo tmutil listlocalsnapshots /.
- Next, enter your local administrator password.
- You’ll see a list backups with dates attached. The dates are the key thing you need to remember.
- Next, copy and paste this command into Terminal:
tmutil deletelocalsnapshots <insert one of your dates>. Based on my listings, I come up with
tmutil deletelocalsnapshots 2018-03-29-154852.
- Once you press Enter, it will be deleted. Once you do that for all of your listings, you should be able to run
sudo tmutil listlocalsnapshots /in Terminal and see an empty list.
Once you are finished, you should have additional free space in macOS High Sierra.