Late 2014 iMac - External Drive Boot Issue

[replicon1]

I'm stumped by this problem with a friends iMac (Big Sur on a Retina 5K, 27", Late 2014 with a 2TB fusion drive and the R9 M290X 2GB GPU). He has constant, random freezing, buffering and crashes so I assumed the fusion drive was on it's way out so I backed up the drive using Superduper (latest version) but when I tried to boot from the newly backup up external drive it doesn't appear in the list of available boot options on start-up.
I then tested the backup drive using a Macbook Air running Monterey and can boot to it no problem so assumed that there is a some security option that prevents booting off external sources on the iMac so I opened recovery and checked the Startup Security Utility but that wasn't even enabled. I assumed that you had to set a password before you could then see the additional settings to allow or deny external boot options but even after enabling this, the only options that are available is to change the password or turn it off. The additional menu options that I see in screen shots of Startup Security from other Macs are missing.
Has anyone seen this issue before?

 

[Fishrrman]

You didn't tell us WHICH VERSION of the OS you're using.

What you could try:
- Try booting the iMac to INTERNET recovery:
Command-OPTION-R
at boot.
- If you can get booted, connect the external drive
- Open the OS installer
- Try re-installing the OS onto the external drive.

Perhaps this action will correct whatever problems that kept the OS from "being recognized" as bootable by the iMac.

Hmmmmm...
WHAT KIND OF ENCLOSURE is the drive in?
WHAT KIND OF DRIVE (who made it, is it an SSD or HDD)?
If the drive is removable from its enclosure, try a DIFFERENT enclosure and cable.

 

[replicon1]

Apologies, that reply didn't seem to work

 

[replicon1]

You didn't tell us WHICH VERSION of the OS you're using.

Thanks fishrrman, responses below:

You didn't tell us WHICH VERSION of the OS you're using - It's Big Sur 11.6.2

What you could try - I took another known working HDD (same spec & model) and went through internet recovery on the iMac, re-installed the original OSX that came with iMac (Yosemite) and again this isn't recognised as bootable drive but after connecting the caddy to the MacBook Air it is recognised as bootable, as with the cloned drive which boots just fine on the Air too.

WHAT KIND OF ENCLOSURE is the drive in? USB 3.0 Dual Bay 3.5" SATA External Hard Drive Docking Station

WHAT KIND OF DRIVE - It's a 3.5" 2TB HDD (internal) can't recall the manufacturer

If the drive is removable from its enclosure, try a DIFFERENT enclosure and cable. - Don't have another enclosure but I have tested it on another devices and it works just fine.

 

[pshufd]

Does it show up in Settings -> Startup?

 

[DeltaMac]

If you have Yosemite loaded on the internal drive, you won't be able to see a Big Sur boot drive. Yosemite does not know what an APFS volume is, and will ignore Big Sur completely. There is no way to set the Big Sur volume as a Startup Disk while you are booted to Yosemite. The only way to boot to the external Big Sur drive is to boot while holding the Option key, then choosing your Big Sur boot drive from that screen.
if you want Big Sur to boot without holding the Option key each time, you can set the Big Sur as the default boot drive by selecting it in the Startup Disk pref pane. Again, you can only do this after booting to the Big Sur volume - you won't see the Big Sur volume at all if you boot to the Yosemite volume.

 

[replicon1]

If you have Yosemite loaded on the internal drive, you won't be able to see a Big Sur boot drive. Yosemite does not know what an APFS volume is, and will ignore Big Sur completely. There is no way to set the Big Sur volume as a Startup Disk while you are booted to Yosemite. The only way to boot to the external Big Sur drive is to boot while holding the Option key, then choosing your Big Sur boot drive from that screen.
if you want Big Sur to boot without holding the Option key each time, you can set the Big Sur as the default boot drive by selecting it in the Startup Disk pref pane. Again, you can only do this after booting to the Big Sur volume - you won't see the Big Sur volume at all if you boot to the Yosemite volume.

Thanks for the response. Unfortunately, again I think I've failed to clarify the situation. The internal drive still has Big Sur. There are two external drives, one with the cloned copy of the Big Sur internal drive with all the data, software etc and the other has Yosemite which I deployed to it using the reinstall OSX option from Internet recovery menu (the version of OSX the mac originally had).
Neither of these drives appear as bootable options when starting the mac but if connect the caddy to my macbook air which is running Monterrey they do and I can boot successfully to them. So the question is what is preventing the imac from seeing them as bootable options?

 

[DeltaMac]

The mystery of the drives that are bootable on one Mac, but not another!
The startup security options will only show the choice of a password (the firmware, (or EFI) password), or no password (which means that the firmware password protection is turned off). The only Macs that show the OTHER options for startup security are those with T1 or T2 security chips, or M1 processor. Your 2014 iMac has neither, so you only have the option of the firmware password. And you want to leave that option OFF (no password) until you get the macOS install, with bootable drives that actually work!

Try a reinstall of Big Sur:
Boot your iMac to an bootable Big Sur installer. A USB thumb drive would work very nicely for that bootable installer.
Select Reinstall macOS.
Choose your external drive (the one that you used to backup the internal drive) as the destination for the install, and reinstall Big Sur on that external drive. (this reinstall re-loads the system components, there is no danger of losing any of YOUR files, unless you choose to erase the drive first. You don't need to try an erase -- yet, just a simple reinstall.