Chosen Solution
Hey there, i have a weird problem with a MBPr 15" Mid 2012. Lets call it MBPrA. MBPrA has a broken SSD but boots normally from an external drive. I removed the SSD from a non working model from the same year (MBPrB) and put it in MBPrA. MBPrA doesn’t boot and i get the ?-Folder sign. When i boot MBPrA from external again the new internal SSD is recognised normally and i can browse files on it. The SSD definitely has a system on it. I’m really really confused… I also need to mention that the SSD is APFS formatted on High Sierra. MBPrA hasn’t been upgraded in a while and might last have run on 10.11. Is it possible it didn’t get a firmware upgrade that is needed for APFS? Thanks in advance!
The drive needs updating (newer APFS). Your systems is expecting the newer APFS file system from what it has in its NV-RAM setting from your other drive. From your external bootable drive re-install the OS onto the older drive. That should fix it.
