I have a 2015 iMac 21.5" that has slowly ground down to crawl. I had this great idea of installing macos on an external SSD (Kingston 2TB) and run it from there instead but no matter what I try I can’t get it to work.

At first I downloaded the installer for Monterey but the HDD isn’t great and it kept crashing and restarting partway through.

Then I tried installing to the SSD from Recovery (internet recovery wouldn’t get past wifi choice so had to use non-internet) but even though the installation went ok it continued to boot from the HDD and in Startup Disk the SSD wouldn’t show at all.

Then I made a USB boot stick and did the same steps above with the same result.

The SSD has GUID map and I tried both Mac OS and APFS formatted.

Does anyone have an idea what might be going wrong? Thanks in advance!