Installer flash drive won't show up in Start Up Disk Preference

I remembered that I had an old SuperDuper! backup, I just now plugged it in, and it was recognised in the Startup Disk Preference. It's restarting now. Maybe I can reinstall the OS from there?

Thanks for your reply.

"However, maybe the USB stick used is bad or not compatible with booting a Mac." I suppose, but it shows up on the desktop, so at least it's recognised.

"Verify you created the bootable macOS USB installer using the instructions in this Apple article:..."

Yes, that's how I made it.

"You may want to try running the Apple Diagnostics to see if any hardware issues are detected." Startup while holding the "D" key produces a spinning globe with an exclamation point in a triangle, with a request for my network that it can never seem to connect to (doesn't recognise the password, though it is correct.)

"you can check the health of the drive(s) by using DriveDx" The report looks totally fine. Here it is attached.

Any ideas?

Thanks for your reply.

"A bootable USB Installer does not show up in Startup Disk." It has many times in the past for me.

"You must hold down the Option key on startup." When holding the Option key at restart it boots directly into the user selection screen.

"Startup Manager" You mean the Startup Disk Preference?

"isn't configured correctly" I made the bootable startup flash disk strictly according to Apple instructions with the Terminal commands.

"isn't recognizing the drive at all." But the flash disk shows up on the desktop, so it is recognising the disk to some extent.

"You don't need to start at 12.0.1." Perhaps, but when I updated it from a much older OS a few days ago, it updated in steps: first to 12.0.1 and then to 12.6.5. I still think there is a chance that a bootable installer with 12.0.1 could be successful. But it seems impossible to find 12.0.1 online.

The Option boot method just doesn't work for whatever reason (nor has any other 'key combo while starting' worked either, and I've tried Option, D, R, and maybe others.) So if the drive doesn't show up in Startup Disk Preferences I don't know what to do.

But I will try another flash drive.

The results of the test are in the "Additional text" feature, as it puts this message length beyond acceptable limits, apparently. I don't know how to access that from your end, but there must be a way.

Thanks again for your assistance.

Do you think that it will make any difference? When I updated the OS after nearly three years of sitting mostly idle, it first updated the OS to 12.0.1, and then upgraded in a second stage to 12.6.5, so I was thinking that maybe the hardware can't handle 12.6.5 directly. But would 12.6.2 really be much of a difference?

Thanks again for your input.

"(I'm assuming you see the Apple boot picker menu even if no external bootable volumes are seen)."

Actually, no. It never gets to the boot picker menu regardless of what key combo I use during start up, including the option key while starting up. It either goes directly to the user selector screen, or else in some cases (it's not completely repeatable) I get the spinning globe (with or without a "!" in a triangle) and a request to select network, which never accepts my password even on the rare occasions that it can see my wifi network.

I've tried to link the DriveDx report in the Additional Text feature; I don't see any other way to link the report since it's too long for this box.

I've been thinking of maybe connecting it with an ethernet cable to another Mac, and trying to install the OS by way of Target Disk mode. Do you think that would work?

"I'm guessing Recovery doesn't work, either?"

Recovery is holding the "R" key down while restarting, right? Yeah, tried but it either brought up the user selector screen or the spinning globe, I don't quite remember which, but in any case it didn't work as it should.

What do you think of connecting this iMac to another iMac (a newer one) by way of ethernet cable and trying to install the OS by Target Disk mode?

Right. I just tried it Command + R and got the spinning globe that refuses to accept my wifi password.

Do you have an opinion on whether it would be possible to reinstall from another, newer iMac by way of ethernet cable on Target disk mode?

Thanks for your reply. But that doesn't work. It boots into the user selection screen.

Right. Thanks for the input in any case.

Interesting. How did you boot from it then? Option key upon restart?