iMac Starts up and shuts down in single user mode on it's own.

Since no one's posted regarding this, and in the event someone might experience the same problem, here's how resolved it. I can get around most Apple tech problems but I am by no means tech-savvy or know that much about this stuff so feel free to correct anything I get wrong. I am theorizing my problem stemmed from my external SSD drive not having a boot partition? How this happened just making an OS update I'm not sure. I tried to fix this via Disk Utility but couldn't get it to work and couldn't figure out how to get to the boot partition after installing Mojave. I then remembered my long-time backup application SuperDuper could clone a drive and also create a bootable disk.

So that's how what I did and it worked. I told SuperDuper to reformat the SSD, make it bootable and then copy my original internal HD to the SSD. Took me two days to figure it out but I have my speedy external, SSD running the show again.

Update: I figured out part or possibly all of the issue. My SSD drive is an external drive and although it worked before perfectly, I think updating it to Mojave made it an APFS format. I guess when it did this it changed the permission of the drive so now it boots up as a drive I can't change or add username's read/write permissions.

I've tried changing them booting up from my original internal HD but it doesn't work.

I've reformatted both drives and re-installed the Mojave OS but that didn't work either. I'm at a loss. I regret the upgrade now. It was working beautifully before. If anyone has any suggestions, I'd really appreciate it.

If I boot up with the external SSD plugged in, my screen is blank for a very long time before it defaults to Single User Mode. Then it reads out a long list of commands and strings I can't keep up with after settling on some lines that tell me the disk is not read/write and to mount it enter the mount command or to boot the system enter exit. I enter exit and it spits out more code and then launches my OS Mojave where I can use it.

I can install apps and use it normally but if I restart or shutdown I have to go through the above sequence again. Also when I shut down or restart, screen goes black then it goes back into Single User Mode, spits out some code and finally shuts down or restarts.

Thanks for taking the time to read this and I hope someone can offer some kind of fix short of buying a new computer. I guess I can go back to the original HD but it was running so slow which is why I added the SSD.