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

Tried to install 10.15.3 Catalina clean onto new Thunderbolt 3 SSD drive. The installer is on an external USB drive.

The SSD drive light was on but there was no progress on the progress bar after several minutes. I've tried three times with same result. Installer created a macOS Install Data directory and restarted the iMac (10.14.6 with today's updates).

Any suggestions? I've sent a problem report to the manufacturer(OWC).


Here's the info on my iMac:

Model Name: iMac

Model Identifier: iMac18,2

  Processor Name: Intel Core i5

  Processor Speed: 3 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: 180.0.0.0.0

  SMC Version (system): 2.40f0

  Serial Number (system): C0****1G9

Hardware UUID: 6B0A21D1-BC8F-5F37-B43E-512E6765C0C4


And the new drive shown in Disk Utility




[Personal Information Edited by Moderator]

iMac 21.5" 4K, macOS 10.14

Posted on Jan 28, 2020 4:02 PM

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Posted on Jan 31, 2020 5:13 AM

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.

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Question marked as Top-ranking reply

Jan 31, 2020 5:13 AM in response to ozarkcanoer

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.

Jan 29, 2020 4:12 PM in response to dialabrain

@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.


Jan 29, 2020 5:34 PM in response to ozarkcanoer

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.

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

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