iMac won't boot from external USB SSD drive if another external USB drive is connected?

I have a iMac (21.5-inch, Late 2013), 2.9 GHz Intel Core i5, 8GB 1600 MHz DDR3, NVIDIA GeForce GT 750M 1024 MB. 1TB (5400-rpm) hard drive.

I stopped using the internal 1TB (5400-rpm) hard drive, it was just too slow. I have been running macOS 10.13 (High Sierra) from an external USB SSD for about a year with no problems. I've had an external USB HHD connected to use as a backup with Time Machine. I recently connected another external USB HHD and installed macOS 10.14 (Mojave) on it (bad idea I know, I though it was an SSD, my mistake). I booted from the Mojave drive, everything seemed fine, then checked for updates. It installed the updates, then when it restarted it when into Recovery Mode.

So I tired to boot from the SSD with High Sierra, but it would also go into Recovery mode. Then I noticed in Recovery Mode where it says Reinstall macOS it had the icon of Mojave, not High Sierra. If I hold Command + R during restart it goes into Recovery Mode and then the High Sierra picture is next to the Reinstall macOS option. If I remove both external USB HHD drives (the backup and the Mojave boot drive) the computer will boot into High Sierra from the external USB SSD drive just fine, however...

THE PROBLEM IS: I cannot boot from the external USB SSD drive into High Sierra if I connect another external USB drive to the computer, so I cannot connect a drive for backup use. I'm guessing that the problem is firmware-based, but not positive. I read that the Mojave installer, which I downloaded from the App Store, includes a firmware update. I'm worried the the Mojave firmware update is affecting USB external drives when I try to boot into High Sierra

I TRIED:

• Reinstalling High Sierra on external USB SSD drive by holding Command + R during restart and choosing reinstall macOS.
• Rolling back the Mojave external drive with Time Machine to High Sierra.
• Reset the PRAM.
• Unplugged everything and reset.
• I installed a fresh Mac OS10.13 on yet another USB HHD Drive, booting from that one was no different.

So Essentially I'm trying to figure out why I can't boot into High Sierra from an external USB SSD drive while also having an external USB HHD drive connected, like I could prior to messing with Mojave.