MacBook Pro 9,2 can't read an SSD with a macOS High Sierra OS
Okay, if you have a macbook pro that works on an older OS (pre High Sierra), and you want it to boot from an SSD (SSD1) that has a High Sierra, you have to make it read an SSD (SSD2) that has Sierra at the most, and then upgrade that to High Sierra.

Once the upgrade is done, you can slap on SSD1 and your laptop should now boot on that drive.

I'm not all too sure what happens here. We just jumped the gun and updated to High Sierra and it fixed the problem. We aren't so sure if installing the firmware updates would have helped.