iMac 2017 27" will not boot from internal drive.

Hi all,

Here is my situation and I will try and give as many details as possible. Below are the specs.

The other day I went to restart and it hung on start up. I then tried every trick in the book including removing some .kext files in terminal and nothing worked. At the time the mac was running Catalina.

After booting in recovery, there was not enough room on the drive to install a new OSX and my time machine backup was from 2 months ago and there were files that I needed after that time (my bad on both, I know!) Actually I was doing a TM backup when this happened.

Eventually I mounted a new 3T HD (formatted as Mac OS extended (journaled), GUID partition map) to the usb-C port with an expansion chassis. I was able to do the time machine backup on that and start up the iMac with the 3T HD as the system drive (with the original Macintosh HD still in the computer) Luckily, the previous system drive did mount (as read only) and I was able to salvage all the files I needed post my last TM backup (which was still at high sierra at the time)

My question is this: When I decided to put the 3T working system HD inside the computer as a system drive, the computer won't boot. Also, in recovery mode disk utility will not recognize it as mounted system drive.

Any ideas?

Thanks in advance!

Dan

iMac   Model Name: iMac

 Model Identifier: iMac18,3

 Processor Name: Intel Core i5

 Processor Speed: 3.4 GHz

 Number of Processors: 1

 Total Number of Cores: 4

 L2 Cache (per Core): 256 KB

 L3 Cache: 6 MB

 Memory: 16 GB

 Boot ROM Version: 179.0.0.0.0

 SMC Version (system): 2.41f2