Chosen Solution
I recently was given a MacBook Pro that had a ram issue. Once I figured out it had a bad ram stick, and replaced it, it gave me a flashing folder error. On further inspection using a SATA to USB adapter, I found that macOS had been installed and used on this machine, on the SSD at one point. I have formatted the drive and tried to install macOS on it with Internet Recovery mode and a High Sierra bootable USB drive, but neither disk utility or terminal will recognize the SSD.
Update (03/01/2020) So I installed the new hard drive cable, and the ssd shows up and is able to be formatted but… when I install MacOS on it. I get a no entry sign. I have tried installing MacOS from a USB drive on High Sierra with APFS format and from internet recovery with Yosemite on MacOS Journaled. Both do the same thing. Something else to note is that it tells me to “reinstall” macOS. But no OS is on the drive after formatted.
You either have a bad SSD or the HD SATA cable is bad or damaged. I’m suspecting the HD cable is bad and will need replacement. Here’s the needed part MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable a common issue is the rough aluminum case damages the cable so you want to place a strip of electricians tape on the uppercase were the cable crosses Your Hard Drive Cable Is A Ticking Time Bomb. The second issue is damaging the cable in the process of putting it in. Often I find people make a hard crease where the cable needs to be routed around a bend. You don’t want hard bends as that damages the thin foil wires in the ribbon cable instead you want a nice arc. To help in forming the arc I use an old BIC pen ink straw using it as a form brake. Bending the cable around the radius as needed. You’ll need to reformat the drive and install a fresh copy of macOS as you want to make sure there aren’t any corruptions.
