Chosen Solution
Hey everyone, First of all I have checked most of the answers people got with the same issue. Unfortunately nothing worked for me. I have recently upgraded my HDD with a new SSD (WD Blue - 1TB 3D NAND) on my Macbook Pro 15” Mid 2012. The first thing I have tried was to connect my old HDD with an external adapter and restore the Mac Partition (High Sierra) on my new SSD. After it was finished it booted without a problem. I later decided to upgrade to MacOs Mojave, and I got the very first Prohibitory sign. It was loading up to certain point and was showing the prohibitory sign after a while. Interestingly, when I installed the HDD back to the computer, it was booting perfectly. So I thought, maybe if I format it with the the new format I am not familiar with (APFS), it could work. But it didn’t. I also tried to clean install Mac Os Mojave to the new SSD but did not work. I finally decided to install MacOS Mojave on the SSD and transfered everything from the old HDD with the migration assisstant. And it works perfectly with the external adapter. After a while my internal HDD also stopped working. I know get the same prohibitory sign with my old HDD with High Sierra. So, I decided to replace the SATA cable, but it did not change anything. I still get the prohibitory sign on my SSD and HDD if the are internally connected. But they work if they are connected via external adapter. Here are a few interesting details: I have a bootcamp partition on my HDD and Windows boots perfectly, internally or externally connected since the beginning. I am almost sure the new cable is working fine, because my infrared receiver was not working anymore, and it works now with the new SATA cable. I am beginning to think that this is really a problem about EFI or the new APFS format, which I know nothing about. Any help would be appreciated. I am booting with an external for the last few weeks now. Thank you..
While Apple states it supports 2012 MacBook Pro’s, I’m not sure if the Unibody was what they meant as there is the 2012 retina model as well. But, if you put that aside, I don’t recommend using High Sierra or Mojave on SATA based system with APFS as the queue depth is not very deep unlike PCIe/AHCI or PCIe/NVMe blade SSD’s used in the retina models. So what to do here?? There are a few different versions of the HD SATA cable and you really need to newer one with a SSD drive. This has to do with the ability of the cable to handle the higher flow rates the SSD pushes and pull’s, unlike an HDD. The other factor is many people damage the cable just putting it in! Many people stress the cable when they fold it around the corners. In truth you want a even arch not a sharp fold as you damage the thin foil wires and the shield. Here is the correct cable: MacBook Pro 15" Unibody (Mid 2012) Hard Drive Cable Apple P/N 923-0084. I also place a strip of electricians tape on the uppercase where the cable crosses over as the rough surface tends to wear the cable as well. I would reformat the SSD and stick with Sierra. Once you get it working with it make a good backup (TimeMachine) just in case. Then give Mojave a try again. It you want to you can alter the installer not to upgrade the SSD to APFS file system.
You want to stay away from APFS in general, so format your internal drive into Mac OS Extended (Journaled), then try installing Mojave or HS onto your internal drive formatted into MOSEJ format and see if works with the drive scheme being MOSEJ instead of APFS.
