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..