Installing Catalina to external SSD


I’m trying to install Catalina on an empty external 500gb SSD, formatted APFS. My Mac is also running Catalina. I went to the App Store and fetched a copy of the installer, which is now sitting in my Applications folder, all 8gb of it.

I start the installer, point it at my external SSD, and it starts chugging, looking all OK. The progress meter says 5 minutes. After around that time, with nothing unusual reported, the screen goes black and the restart sequence begins. But it restarts from my internal disk and the new installation is not complete.

Second attempt, I followed instructions to create a bootable USB installation disk on another USB drive, booted to it and started the installation from there. Same result.

Any suggestions?


I suggest that you make a bootable installer from your downloaded Catalina installer app, boot to THAT, then try your install on the external drive.
There's lots of methods to do that, but I usually just use the terminal. If you need some help with that:
I made some notes about that a few months ago, and is as simple as I could make it (outside of using one of the dedicated apps that you can find.)
Make installer bootable on flash drive:
Insert your USB flash drive (use a 16GB flash drive for this). Erase that thumb drive, using your Disk Utility. Make sure it is formatted Mac OS Extended (journaled), and the partition Map is "GUID"
The Catalina installer app (Install macOS Catalina) should be in your Applications folder.
Right click on that installer app, and choose "Show Package Contents". Open the Library, then open the "Resources" folder. Leave that alone for a moment...
Launch your Terminal app, then type sudo, and add a single space.
From the Resources folder that you left open, drag the file "createinstallmedia" and drop it on your terminal window. You will see the full path to that file appear next to sudo.
Next, type --volume, and add one space to that. (notice there are TWO dashes, not just one)
Now, find the icon for your flash drive (the one you just erased. Should be on your desktop somewhere). Drag that icon into the terminal window. You will see that disk, with its path, added to the line you have been typing.
AND, press enter. You will be asked to enter your admin password. Do that, you will NOT see what you type (it's a security feature, just type your password, then press enter. It will work.
Finally, the terminal will ask you to press "y" to start the command that you just typed, so type Y, and press enter.
Then, you will see some numbers appear, showing the progress of the command. Some flash drives will be slow, maybe 30 minutes. It will get to 100%, then do a few final items to complete the installer and make it bootable. And, it's ready to go!
Choose your external drive as the destination for that install, and that should do it. The Catalina install will automatically make the partition APFS, so there's really no need to go beyond a simple HFS extended format.
Also, don't forget that the installer in your Applications folder will delete itself after a successful install, so that's the main reason I suggest the external bootable installer.


Thanks, DeltaMac. This all works now, because I learned the hard way an important lesson about my late ‘13 iMac and it may have wider scope.

Thou shalt not boot from an external drive unless it is plugged into a USB 3 port on the system itself.

I had my drive plugged into a powered USB 3 hub. Any restart just fell back to the internal disk. :rolleyes:


Ah, good plan!
Not all powered hubs are the same: I have a cheap (Insiginia brand) 4-port hub that I usually use bus-powered. I also have a Late 2013 iMac, same as yours, booting through the hub from a cheap 256GB external SSD. I use the same hub to install and boot to other systems, and that hub, (which I bought because it was cheap) works without missing a beat.
However, plugging a boot device directly into the system (no hub), is one of the items that I check for when troubleshooting boot issues.