MacBook Pro 2012 SSD issue

You neglected to mention the make & model of the "original" SSD that came with the laptop.

Assuming the replacement cable is good (no idea where you purchased it or whether it may be a used cable), then more than likely the Logic Board is bad since the SSD works fine when booted externally. There is still a slight possibility that you have two SSDs that are just not compatible with this laptop.....not all SSDs are compatible with all computers or devices.

I would probably try writing zeroes to the beginning of the SSD to destroy the partition table & any hidden items at the beginning of the drive. Then use Disk Utility to erase the whole physical SSD & reinstall macOS Catalina again. I would suggest doing this while the SSD is connected externally to make sure everything goes well & so that nothing is corrupted by a potentially bad cable or SATA controller. Then reinstall the SSD internally to see if it will boot. Here is the Terminal commands needed to write zeroes to the beginning of the SSD, just make sure to replace "diskX" with the correct drive identifier for the SSD you want to erase:

diskutil  unmountDisk  diskX

sudo  dd  if=/dev/zero  of=/dev/diskX  bs=100m  count=10

The second command will prompt you for your admin user password. Nothing will appear on the screen as you type the password, so just press the "Return" key to submit the password.

Keep in mind you may have just discovered the reason the previous owner of this laptop got rid of it.

mdilsara wrote:

I bought a MacBook Pro 2012 recently, it came with a 1TB SSD drive and it booted into flashing ? mark whenever I try to turn it on. After my research I assumed it was due to the faulty SATA cable and ordered one. Even after replacing the cable, it doesn't show any partitions as when I turn it on with option button pressed. I tried booting into internet recovery and SSD does show in the Disk Utility, but it doesn't allow me to perform first aid anything on the disk. I have already installed MacOS Catalina on the SSD and it boots fine with an external enclosure. It followed the same with an another ADATA 480GB SSD and came across same problem.

A normal 320GB HDD drive boots fine attached to the new SATA cable as well as with the old SATA cable. What could be the issue? Is it still with the cable or logic board?

Those 2012 MacBook Pros never came new with 1 TB SSDs, so the previous owner had already opened it up to replace its original internal drive. Various cables and brackets and other connectors are easily damaged in that tight space when this type of "surgery" is done, especially as you don't know the skill or qualifications of the earlier owner(s). You replaced the cable so it seems most likely that the logic board is bad. Which means the laptop has other problems as well. As HWTech noted, this may be why the previous owner unloaded it. I truly would never purchase such an outdated used Mac for this reason.