Trouble doing clean install of 15.3 onto new external Thunderbolt 3 SSD

Problem solved with assistance by OWC. I used Disk Utility to Erase the Media level of the device. That removed 'corruption' reported by Disk Utility First Aid.

I was then able to clean install 10.15.3 onto the external Thunderbolt 3 SSD. I can boot from the drive to use Catalina and switch back to my 14.6 system drive. The red-herring was if it was an issue doing the install from USB drive or internal system drive. Both failed because of the problem on the new drive.

Now I can test my applications on Catalina before migrating completely.

@dialabrain, I tried installing 15.3 directly from the iMac instead of a USB thumb drive. That failed. Also tried 14.6 directly from the iMac and that failed also. They had the same behavior - after the installer had copied the necessary files onto the external Thunderbolt 3 SSD and restarted the iMac the Apple logo screen showed a progress bar that never went further.

I ran Disk Utility First Aid on the APFS setup of the external drive and it comes back saying there is corruption and to run First Aid from a Recovery boot. So I did that and it did not remove the corruption.

I've reported this to OWC with all the details and asked about a replacement device.

I am trying one more approach. I am using Carbon Copy Cloner to clone my iMac boot SSD to the Thunderbolt drive and then will see if I can boot from the Thunderbolt drive. If that works or fails I will contact Apple Support for their help.

Then I'll give that a go next. I've got a clone copy of my Macintosh HD from the iMac. (two actually on separate drives using two different softwares (CCC and SuperDuper) and one drive is a HD and the other a SSD).

Thanks for replying both times. I'll report after I do it tomorrow.

Cloning my 10.14.6 system drive to the external Thunderbolt drive with CCC worked! And I was able to boot from the external and everything worked as it should.

So despite the Disk Utility thinking that there is corruption on the External SSD it now boots just fine with 10.14.6. CCC just updated the external.

So what is strange about doing an install of macOS (14.6 and 15.3 both fail) to that Thunderbolt external drive? Will report to OWC and to Apple.

My next step is to see if a upgrade install 15.3 on top of 14.6 on the external will work.

Would correct the problems I'm having? I was able to select the new drive for the installation.

Thanks Moderator. Didn't intend to reveal personal stuff that shouldn't be out there.