External Boot drive not appearing in boot manager

[GorillaPaws]

I have an older iMac (2011ish) running El Capitan that has a hard disk that's failing. It won't boot normally. I was able to successfully transfer my files via targeted disk mode to a backup disk. My plan is to run macOS High Sierra on an external SSD via a thunderbolt drive array (Thunderbay 4)--This array also happens to contain a 3.5 drive as well (likely irrelevant, but I wanted to mention for completeness). I have successfully installed High Sierra on the SSD and confirmed that I can boot from it via the boot manager on a Mac that's working normally. When I attempt to boot from the external SSD via the boot manager interface on the broken Mac I'm only seeing the internal disks: the Hard disk, the recovery partition and a bootcamp partition. I have tried this using both thunderbolt ports with the same result. I have also tried resetting the PRAM.

Is there a technique to compel the boot manager to locate the disk? terminal commands?
Could this be an issue with the Mac's os being El Cap and my external ssd having High Sierra?
Any other suggestions?

Thanks in advance for any help.

 

[Fishrrman]

If the startup disk preference pane CANNOT "see" the drive you want to boot from, then I would reckon it's either:
1. The OS on the drive isn't capable of booting the Mac
2. The OS on the drive is corrupted, or maybe it isn't there at all
3. There's something wrong with the drive's drivers at a lower level than the OS
4. There's something wrong with the enclosure the drive is in.

I'm going to _guess_ that the fact that you're using some kind of "array" -- and NOT a single drive in a thunderbolt enclosure -- may have something to do with this.

Do you have a USB2 enclosure by any chance?
Have you tried putting the drive in question into it?
(Yes, I know that booting from USB2 will be slow, but it may work, whereas the Tbolt array doesn't)