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 &amp; startup key combination, it returns to the update installer.

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