Can't boot off of an external drive.

Well, here's what happened. I need to physically disconnect both my Lacie and OWC drives and Opt-boot to get the above display. Other drives plugged in don't matter. Also, without Option during the boot, it boots off the Untitled drive (internal SSD that was part of the original Fusion disk, whose HDD died), which I don't want. Why those two drives mess it up, I don't know. Why I have to Option boot, IDK. For some reason the "Untitled" is the default one, even if both come up. Also, I tried setting the Startup Drive in Sys Prefs. Worked, but only once! It then went back to both disks being light and neither selected.

Tip: How to tell which is your startup drive: Open Finder. Click iMac or whatever your machine is called. You should get a list of your volumes. Add the Kind header and column. (Right-click an existing header and check Kind on the pop-up menu.) And viola`! It will tell you which is the startup drive!

Just rebooted. Got only the tiny internal drive "Untitled". Did a complete shutdown. Then Option-booted. Still only the internal drive. Shut down. Disconnected my backup drives. Option-booted -- then it worked! Got my external boot drive. See photo below. Note: auto-exposure overexposes. I turned it down a tisk. Much better. So it appears to be the backup drives that are messing things up. Next time I'll disconnect them and then do a regular reboot.

I forgot to mention that when I boot with the option key I get a choice of one disk. It’s called “Untitled” and it’s the SSD drive. I migrated my applications and computer settings to this small drive of only 128 GB. Still I think it be easier if I could boot off the second hard drive.

I finally got it to work! Turned out the new external drive (Macintosh HD2) didn't have a full Mojave installation on it. So I erased it and called it Macintosh HD2. At some point I disconnected everything else possible, even the printer! Tried running the .dmg on HD2 manually twice. Gave up. Did a reboot with Cmd-R. Let go of the Cmd-R early due to changing my mind. Then when it came up, it started installing Mojave on the 2nd external drive, Macintosh HD2! When it was done I used Migration Assistant to move files from the old external HD, Macintosh HD, to the new one. (I initially tried my TM backup, as it's on a healthier drive, but it came up with only a backup of the SSD drive. So I thought I would try the HD drive first. [It has backups of HD, but not in the latest folder. Would have to experiment with alias Latest. Also, a recent TM restore from that drive put files on the old HD missed all the files in the Pictures folder and messed up the music files.] So I chose to go with the old external drive, HD, thinking it could die at any time and I would thereby lose an option.) Took 13 hours, but it worked! Had to go through some set-up. But now I have a working system without having to wait for an SSD to be delivered. The old drive was really slow, but the migration completed anyway.

Sure, buy an SSD. I keep hearing that as if they were cheap. NEWS FLASH: They're not!

OWC has a 4TB SSD for $929. Crucial has one for half that. Why?! What benefit would there be to the OWC one? I like OWC, but what's with the price?

Anyway, I can now finish my video format conversions, reduce the size of my boot drive by offloading the videos, and move to a new Mac and also get an external SSD for the videos.

Yes, the internal HD had completely failed. Erasing it would come up with a problem accessing the last block. The senior tech agreed: it was a goner. The SSD worked fine though!

I tried the DeviceDX program someone here recommended. It said the SSD was at 14% of its rewrite life! It also said the old drive was just fine. I ran the short self-test on the old drive (HD) and it got up to 90% and hung there. I waited 15 or 20 minutes or so and then aborted it. The app then said the test completed successfully! My last run of EtreCheck said the drive (HD, the boot drive at the time) was really slow and might fail at any time.

Now things are working pretty well. I'm not sure if there really was much benefit from the SSD of the Fusion drive.

Anyway, I'm not dead in the water anymore! I have to set up backups again, but need to be careful to inherit things right!

It appears that either powering down or disconnecting all other drives does the trick. Anyway, I finally have a working system using the new external 4TB SSD drive. Now to finishe the last few video format conversions (the least important ones) and offload enough movies to make it fit on a 1TB drive on a new M1 Mac. (512 GB seems a little cramped, and 2 TB is way too expensive and not big enough anyway!)

It might have. I know the NVRAM has startup disk information. I tried it at one point but I don't think it made any difference. Even if it did, I can't be resetting it every time I boot!

I just need to disconnect those two disks and boot with the Option key. I suppose I could set the startup disk in sys prefs whenever I need to reboot. That should avoid the need to Option boot. But I still have to physically disconnect those disks.

Whenever I check the sys prefs startup disk page it is always not set to any disk. The two bootable disks are there, but neither is highlighted. I've never seen it do any good with the 2 "rogue" HDD disks connected.

Well, I've cleaned up my startup disk. If I move my Movies to my 2TB SSD drive, and empty the trash, what's left should easily fit on a 1TB drive in a new imac. In fact, I calculate 364 GB. So 1 TB is probably fine. I don't want to pay another $400 for an additional TB on the internal drive!

Already tried that. No dice. I might be able to do my video format conversions as is, well, I might need to move the video files to the other external hard drive.

Hey, thanks for the About this Mac trick. That's even easier! OK.