I'm repairing a 13" MacBook Pro Early 2011 (A1278). This will seem like a duplicate issue, but stick with me and you'll see something odd is going on.
TDLR; Fixed missing/undetected boot drive issue with new SSD and SATA cable, or so I thought. The issue still persists, but I've discovered it only happens when the MacBook Pro is left shut off for more than ~5 minutes! If I shutdown, then reboot within a few seconds or 2-3 minutes it works. If I leave it much longer the boot fails with the folder and question mark icon!
This MacBook Pro has the "question mark on folder" boot issue - here's a picture. This usually indicates a corrupt/failed boot drive or faulty SATA cable. I had the same issue on my own 2012 MacBook Pro a few years ago, and did the same repair (replace the cable and SSD upgrade) so I thought it would be easy.
The repair went smoothly. I got High Sierra installed on the new SSD by creating a bootable USB. Rebooted a few times afterwards to confirm all was well and considered it a job well done.
Unfortunately, when I tried to boot the MacBook Pro later in the day it displayed the error again on boot! I performed an SMC and PRAM reset and then it booted up. I assumed all was well.
I decided to verify my work a little later, and it failed to boot again!
It seems as though the issue occurs if the MacBook is left sitting without power for a while. If I leave it sit on the boot failure screen for a few minutes, then reboot, all is fine. I assume the original issue was never the old HDD or SATA cable, but something power related maybe?
I appreciate any ideas you have.