Don't think I have it in me to help friend with 2015 iMac 21.5" swap HDD to SSD

[GGD]

jdc wrote:
[quote=GGD]
Unless things have changed radically over the years, you should be able to use the Startup Disk system preference to select the external boot drive, and as long as it's powered on, it should boot from it on restarts and power ons.

Exactly. Its always powered on, since its a bus powered drive. =)

Choosing the correct drive at startup requires no more than holding down the option key at startup and choosing the correct drive.
I'm confused by your answers and experience.

You answered "Nope" to can it automatically select the drive, and "Exactly" to my description of how Startup Disk works.

So does Startup Disk work for you or not? Or do you just prefer using the option key.