Strange things happen with a SSD disc on MacBook Pro

You should be able to use Internet Recovery on that machine. Follow instructions here: About macOS Recovery - Apple Support

To manually start up from Internet Recovery, press and hold either of these key combinations at startup:

• Option-Command-R
• Shift-Option-Command-R

I've just encountered a couple of our organization's Macs which won't boot my external Mojave drive. Each time it was due to the system firmware being too old. In order to boot a macOS 10.13+ drive on a Mac the system firmware needs to have been updated with a macOS 10.13+ installer.

Use your macOS installer to install macOS to this laptop and if necessary upgrade the OS afterwards to at least 10.13. You will need a properly formatted (aka erased) internal drive in order for the installer's system firmware updater to work. The internal drive must be erased as GUID partition and MacOS Extended (Journaled) even if you install macOS to an external drive.

HWTech--

When you explain it that way, I think you have hit upon the reason why the working internal drive is a requirement in this process.

The firmware installer will set up a tiny firmware Installer the boot drive, set it as the startup disk, and then boot from it to accomplish the firmware update. If any part does not work completely as planned, no firmware update is performed.

If the new SSD is already in the drive, just try Internet Recovery. That model is supposed to have a firmware recent enough to support Internet Recovery. If that doesn't work, then you can try the 3 steps.

Does the SSD from the laptop you swapped cables with work in this laptop? A different SSD can have different requirements to run and perhaps even the "good" cable isn't really that good for the newest SSD. The weakest link is still the hard drive cable. You should also use compressed air to clean out the hard drive connector on the Logic Board. You should also try your new SSD with the other laptop using the main drive bay. The severity of the failure of these cables is different each time so there is no way to guarantee the cable is good even by swapping drives or cables as each drive is unique and will react differently as well. Only a new cable will tell the truth.

I don't have much faith in using the optical drive bay for an SSD since its controller was never meant to be used for such a high speed device. Of all the Apple laptops the 2012 laptop does have the best & most reliable SATA controller for the optical drive bay (this doesn't mean it is great though, just better than older models where it was terrible). However, you will run into the "missing firmware partition" error again if you upgrade macOS again (not sure about reinstalling the same OS) unless there is a properly erased drive in the main drive bay.

The hard drive cable failure can have several different symptoms. However, usually the cable cannot handle the extra amount of data being transferred with a faster drive like an SSD. An SSD is capable of transferring data 4x faster so it needs a much more reliable cable. Think of the bad cable like a speed bump. If you are traveling slow over the speed bump the car can handle it with a little rocking or jolting, but if you go over that same speed bump at 100 mph bad things will happen to your car.

Starting around 2014 many of us discovered that the SATA ribbon functioned perfectly with a rotational HD but an SSD failed. It was infuriating to have an HD that worked internally, an SSD that worked in an enclosure but didn’t exist or wouldn’t boot when it was installed in the computer.

Glad you got it solved.

First, what model MacBook Pro 13-in, by year, early mid late, processor speed?

Thank you all!