Catalina update 10.15.4 freezes with 5 min. remaining

I’m trying to update, but the installer hangs with five minutes left, and no reboot sequence can get around the installer. It’s not a space issue. There’s ~300GB on the SSD. Pressing the power button once changes the completion time to 1 minute, then 14 minutes, then back to 5 minutes.

I cannot get to the recovery partition, and cannot choose another startup volume. With every reboot & startup key combination, it returns to the update installer.

Anybody else getting this? Late 2012 iMac, 16GB RAM, 1TB SSD.

Solution 1:

Something was clearly borked on the target SSD, as after finding and booting from an external Catalina startup disk, I could reinstall Catalina on the original disk, and all is well. Possibly something with the nested APSF volume chain?