Chosen Solution
My MacBook Pro won’t boot from the hard drive internally or externally, though I can boot from that drive when plugged into another device via USB in an external enclosure. When trying to boot up normally, it shows the flashing folder with a question mark icon. When trying to boot to startup manager, the cursor loads but no drives are shown. This is the case with this drive internally on that computer or when plugged in via USB externally to that computer. I can boot from that drive fine on another laptop, no issue. I’ve tried everything I can find online, resetting PRAM/NVRAM, SMC, safe mode, disk recovery, everything I can find suggested. The fact that it won’t work on that machine internally or externally suggests to me (perhaps wrongly) that it’s not the hard drive cable, and the fact that it can boot using a different laptop suggests that it’s also not the hard drive itself. Does this only leave the logic board, or does anyone have any other ideas of something to try? It’s been hard to find this exact set of symptoms online elsewhere, so any help at all is much appreciated!
Sounds like you are hitting the updated file system GUID/APFS from High Sierra and onward where your system can only access drives which have the older file system GUID/HFS+ (a.k.a. Journaled File System). When Apple updated the file system they introduced new system firmware so the systems could then boot up under the newer file system. Sadly, your system did not get a firmware upgrade so it can’t boot up under APFS volumes.
