Catalina Not Happy With 2012 iMac
TL;DR - Installer Stuck, Multiple Attempts, Recovery Mode, Erased Drive, Installed Clean 10.14 (Old Mojave), Migration Assistant Time Machine Backup Restoration, Back To Working Mojave.
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I attempted to upgrade from Mojave to Catalina. Seemed to be installing slowly but surely. A few hours into the install process I peeked in and saw the iMac was stuck in a loop of restarting, throwing an error 10 seconds later 'Your computer restarted because of a problem. Press a key or wait a few seconds to continue starting up.' and then restarting again only to throw the same error again.
I made sure I had well over 10GB of free space for the system to work with (writing temporary files, moving data, etc). I forced the machine off by holding the power button. I disconnected any external drives (including backup drive). I booted into recovery mode by holding command+r at the boot chime. I tried to install a clean version of Catalina from recovery only to get stuck again, even after running First Aid with the Disk Utility.
I had a Time Machine backup external drive so, still in recovery mode, I launched Disk Utility and noticed there was a new Macintosh-Data volume. I deleted it. I then installed a new clean version of 10.14 (Mojave) onto the normal Macintosh volume (which in my case is the 128GB SSD + 700GB HDD in one logical Fusion volume). The other 300GB of my internal 1TB HDD is partitioned for Windows 10 Pro. I reconnected all my external drives (along with backup drive). I logged into the newly installed Mojave, launched Migration Assistant, then restored from my latest Time Machine backup.
My iMac is back to running Mojave. I will be on the lookout for future updates but let this post be a reminder to those with older hardware (Late 2012 iMac, 3.1GHz i7, 16GB RAM, Fusion Drive, NVIDIA GeForce GT 650M) that Catalina may give you a bit of trouble, and to BACKUP your system with Time Machine prior to trying out this new update.
Cheers
iMac Line (2012 and Later)
