2015 iMac got circle with line through it 2 days after I moved it on boot. Computer just couldn't see the HD. HELP! (Info in comments)

So, a couple days after I moved my iMac I rebooted because it seemed to be acting up and on boot I got a black screen with a circle with a line through it. I looked online for fixes and ended up restarting holding command-r to get into recovery mode. Once in there I went to disk utility to try to repair the HD and it basically didn't see the hard drive. So people on reddit suggested the drive was dead. I'm not so sure about that. Sooooo, I had an external hard drive that I re-formatted using my other older mac and then I reinstalled OSX onto that drive using the original backup to the iMac. Didn't think it would be ideal booting from some random external drive but it works. 

So when I finally booted up from the backup restoration, I noticed that the computer in fact DOES see the internal drive. So the computer was showing the external drive that I'm booting from, it also sees the Time Machine backup external drive AND it saw the internal drive! So I know the computer can see the internal but something isn't right. The internal even showed up in disk utility.

The computer seems a bit laggy overall because I'm booting off an external drive (a small pocket WD My Passport Ultra) which I doubt it meant for booting off of (more for data storage). So I rebooted the computer to see if it will run any faster off the external after the initial reboot from restoration and I got an error saying that one of my drives is unrecognizable and gave me the ability to ignore or initialize. I canceled that and when I got to my desktop, I noticed the computer doesn't see the internal HD anymore. Just the external boot drive and the external time machine drive. And the internal is not showing up in disk utility anymore. It's just showing "disk0s2" (APFS Physical Store disk0s2) for "Internal"

Any ideas on what I should do? It JUST saw the internal drive. But then I reboot and it doesn't show. WTF is going on here?

Any ideas guys? macOS 10.14.6. How can it be working for 2 days after I move it and then I reboot and it won't see the internal drive and then after I restore to a external drive and I boot and the computer again SEES the main internal HD and I reboot again and now it can't see the internal. I don't think I can really use the computer for what I need off this external hard drive. Either I need a better external hard drive to boot from or preferably I'd like to get the internal working right. And at this point, if I need to somehow wipe the internal and reinstall osx from scratch, I'll do that. Thanks!