iMac (13,2) won't boot beyond blank white screen

I'm trying to revive a Quad i5 iMac 27 (13,2 - 2.9 GHz w/24 gig of RAM) that won't boot past a solid white screen. On startup I get a chime, then it sits on the blank white screen regardless of how long I let it sit in that state. When I first restated the computer I got the Apple logo and the progress bar but no progress shown, even after waiting an hour. I forced a shutdown using the power button, but when I restarted it stuck on the white screen. I've tried the Option/Start, reset the PRAM several times. No change. I unplugged it and let sit overnight, but when I plugged it back there was no difference.

Since the machine doesn't get much use I figured the the internal HD had died. I reformatted an external HD and did a fresh install of Catalina on it and nothing else. I'm not sure which OS version is installed on the internal drive but supposedly this (13,2) iMac can run Catalina so I created an external, bootable drive using a similar vintage Quad i7/Mini, downloading Catalina from Apples OS archives. The D/L and installed appears to have worked fine. The Mini booted fine from that "new" external HD. However the iMac still refuses to boot past the white screen so I can select the external HD to boot from.

It seems that the computer is trying to boot but I feel like I overlooking something. But what?

TIA for any help