Installing Catalina on an External Drive

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Installing Catalina on an External Drive

I'm using a late 2013 27" iMac that is running Mojave 10.14.8, and would like to run Catalina from an external drive. I have a T7 SSD and have formatted it as APFS format with GUID partition map. I run the Catalina installer selecting first "Show all Disks" and then selecting my SSD which I named "Macintosh Catalina" during formatting. When the installer finishes it restarts my Mac which boots to the existing Mojave OS. In system preferences under Startup Disk Macintosh Catalina does not show up. Looking at Macintosh Catalina in the finder in the finder I find a single folder named macOS Install Data:

This is not what I expected.

The procedure I used is from an article by MacWorld: How To Install macOS On An External Drive | Macworld It is lacking some important points. It says to use the GUID partition, but never mentions AFPS. Luckily I was aware and did format the disk AFPS.

Where did I go wrong?

iMac 27″, macOS 10.14

Posted on Mar 6, 2023 2:56 PM

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Answer 1

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D.I. Johnson

Level 7

23,795 points

Mar 6, 2023 3:30 PM in response to BobAF6C

Erase or reformat your external drive again as you did originally. When that is done, Disk Utility should display it as an APFS formatted drive with a Container and a single APFS volume named "Macintosh Catalina" as you called it.

Then, download the Catalina installer app from the Mac App Store. It should be found in your Applications folder.

Double-click to launch that installer app and, following the on screen prompts, select your external drive and complete the installation. If you have previously downloaded the installer and it still resides in the Applications folder, you can try using that. When the installer successfully installs the Catalina OS it will delete itself from the Apps folder.

Please see: How to download macOS - Apple Support

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Answer 2

BobAF6C Author

Level 1

14 points

Mar 7, 2023 3:59 PM in response to D.I. Johnson

D.I. Johnson - thank you for the guide. I believe that is what I did. Here's a bit of followup.

This morning I restarted the computer and to my surprise the installation to the external SSD continued. It ended up with two volumes on the SSD: "Macintosh Catalina" and "Macintosh Catalina - Data". "Macintosh Catalina" shows up in the System Preferences "Startup Disk" utility; the other volume does not. Just did some reading and found the second file should be there. By the way, the "Install MacOS Catalina.app still remains in my Applications folder. Most likely it is there because I did the installation on an external drive. I imagine if I had updated the internal drive it would have been deleted when the operation was completed.

I haven't tried to boot into Catalina, as I want to make an extra set of backups of Mojave, just in case.

Thanks again. You were very helpful.

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Answer 3

Mar 7, 2023 4:06 PM in response to BobAF6C

You're welcome. 🙂 It sounds like you've got things dialed in.

Yes, the newer macOS versions from Catalina forward use the APFS format and end up with those two volumes. Usually they are named "Macintosh HD" and "Macintosh HD - Data".

The ...-Data volume houses your user Home folder and data. The other is a secured volume that houses the macOS system files, keeping them from malicious tampering and harm.

Strong work, my friend. Happy computing.

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Installing Catalina on an External Drive

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