Booting older iMac from external SSD

You need to Option Boot the iMac and select the external SSD in order to continue with the OS installation to the external drive. Sometimes the Mac will automatically boot to the external drive during the second phase of the OS install process and other times it will boot to the default internal drive.

If you are running macOS 10.11 or earlier on the internal drive, then you will not be able to use the Startup Disk System Preferences within macOS 10.11 or earlier because those earlier versions of macOS do not understand the new drive layout and APFS file system utilized by macOS 10.13+.

Please always include the exact model of your Mac and also the version(s) of macOS involved as this is extremely important information to have. There are alternative methods if this is the case, but we need more specific details.

Did the macOSinstall finish on the external SSD so that you could actually use Setup Assistant to setup a new user or allow for a migration?

Is the external SSD connected directly to the computer? Have you tried disconnecting all unnecessary external devices in case one of them is interfering?

There is a possibility the external SSD is not compatible (or possibly bad). This tends to be more of an issue with macOS 12.x Monterey though since Macs can be picky about the drives used for booting.

What is the problem you were having with this computer when booting to the internal drive? It is very likely this computer has a hardware issue which was the point in trying a clean install of macOS so as to quickly & easily separate hardware & software issues.

Ratty888 wrote:

Did the macOSinstall finish on the external SSD so that you could actually use Setup Assistant to setup a new user or allow for a migration?

Progress showed installation finished and then it restarted automatically (on the HDD) before giving me the Setup Assistant. I was not able to setup a new user or do any mirgation.

What happens if you Option Boot and select the external SSD? It should allow the second phase of the OS install to continue if the first copy phase completed successfully (assuming the restart was due to the installer and not a hardware issue of some sort triggering a restart). Also assuming that you haven't since erased or modified the external SSD. Like I mentioned earlier sometimes when installing to an external drive the system will reboot to the internal drive instead of to the external drive so you must reboot the system and manually select the external drive. IIRC normally there should be only one restart required when installing macOS, but I also don't pay close attention to the process as I usually focus on other things as it installs. If the Mac is being managed, then it can affect the process which could include multiple reboots (each may require manually selecting the external drive each time).

SSD is connected directly to the computer. The only other connected drive is TimeMachine. I did not disconnect this at the time.

I would never leave my backup drive connected during a major event like installing an OS as I would not want to accidentally erase the wrong drive or worry about something going wrong and causing some damage to the backup drive. I guess I've seen too many things go wrong over the years that I am especially cautious with my storage devices (especially backups).

Having this drive connected may be interfering with the installation process and perhaps may be why the iMac is not restarting to the external SSD during the OS install process.

The problem was 'slow to start' and 'beachballing'. Neither were a huge issue though. The Apple Genius rulled out a hardware fault and he found a few Makware issues that he cleaned up. System is now faster to boot up from the HDD (about 90 to 120 seconds) but I still get the beachballing. The Genius believed that a clean instal would get rid of years of upgrade dross that he thought to be the cause of the beachballing.

The two most common causes of a slow computer are an issue with third party software or a failing hard drive. Unfortunately the Apple Diagnostics (even the service diagnostics) don't detect many types of drive failures. To check the health of the internal drive run DriveDx and post the complete text report here using the "Additional Text" icon which looks like a piece of paper. If your iMac uses a Fusion Drive setup, then post the health report for both the SSD and the hard drive.

To look for possible software issues run EtreCheck and give it "Full Disk Access" so the report will include more details which may provide more clues. Post the report here using the "Additional Text" icon which looks like a piece of paper.

You are welcome. I'm glad it turned out to be that simple to finish the installation.

You may still want to consider running EtreCheck while booted to the internal drive. You can run DriveDx from the external boot drive to check the health of the internal drive if you want.

What external SSD are you using?