Free up disk space during Catalina update I tried updating to Catalina, but it got stuck on not having enough disk space. It says I have to quit the installer, select a start disk and boot from that, but my system's drive is not in the list of drives that I can select...

So, instead, I started the Terminal, went to the drive through that and I started deleting things. But every time I run df I don't see a change in the amount of disk space. Between reboot changes are persisted though, even creation of new files. 

I then installed Mojave on an external drive and mounted the internal drive, it shows up as 52GB free (vs 12GB from df). But when I run df I still see only 12GB free.

I figure there is some sort of place where deleted files are getting moved to in order to provide me with some fancy restore magic for when the installation is done. Where is this magical place and how do I delete files from there so that I can continue the installation?

The answers here don't address my issue because I already know what was eating my disk space.
So, after a few hours of working on this (mainly making rsync backups) I have it working.

What was crucial was creating a new macos installation and mounting the old disk on that. I didn't really delete any files and df kept showing the same amount of free space, but after a reboot to the original disk the install passed.

I think it has something to do with things being purged by the new macos install (Mojave on an external drive).