best Mac OS version for 2012 21" iMac (booting from external SSD, 8 gb ram)

[sekker]

I don't understand the issues with HS or Mojave. Both were good upgrades to my main machines, and they are getting Apple security updates.

Sierra is going to age.

Thunderbolt would be a better way for the external SSD.

[clay]

For this particular user, High Sierra is going to be more than adequate. SSD in a USB 3 case is also more than adequate. For a single SSD on a consumer/home system, thunderbolt would be way overkill and not worth the $$.

Thanks, folks!

[jdc]

Plan sounds solid. I would need to see actual proof that it has malware. $10 says it doesn’t. You have better luck getting struck by lightning than malware.

[clay]

malware/spyware/crap that gets installed when user installs "Clean my Mac/similar" crapware utility to "speed things up"? Yeah, that's what it had. Malwarebytes found and deleted it.

Also discovered that the spinner is failing. That's probably the real reason for most of the sluggishness.

Everything is now complete. 10.13 install is running quite well externally.