I okayed a 10.15.4 Supplemental update (1.38GB) about 3 hours ago, it looks like the 1.38GB had been downloaded but the progress bar remained stucked at about 80% since then.
Brand new mac mini.
Mac mini, macOS 10.13
Well I seem to have had a very similar problem. I did a clean install of Catalina yesterday on my 2018 15-in MacBook Pro. And I used migration assistant to bring all of my data back onto the machine. Everything worked flawlessly. today I tried installing the supplemental update and now when the Mac restarts it acts as though I have a brand new machine with no data on it.
If I boot off of an external drive and look at my internal drive my data is still there in the APFS partition but I can't boot from it without the installer script acting like it's a brand new machine with no data. Very frustrating indeed.
Sounds like a different problem.
if indeed you performed a real clean install, you need to reinstall everything. As you seem to have backup, you can bring back all data.
usually doing so, i prefer to reinstall all programs, then manually bring back just data.
That also gives an opportunity to be selective on what you really want (need) to bring back, and forget all the crap you no longer need.
I could solve the issue just by restarting the Mac
Catalina 10.15.4 supplemental update stucked for hours!
