disk utility: first aid finds no problem with the disk and disk utility shows 16 GB free space on the hard drive
creating a bootable Mojave on a USB drive and booting from that - this works, but it is only a very basic system that only gives me a few options.
booting Ubuntu from a USB drive - that works, but it no use, as Ubuntu can't read the APFS formatted hard drive. I recently performed a clean install of Mojave on my Mac, and when copying back the backed up data my disk filled up and the computer crashed.
