Chosen Solution
Erased my SSD before installing a new macOS 10.14.3. The SSD can be booted as external drive. After mounted it as internal drive and attempted to boot it. Prohibit mark showed up, even original HDD was connected as external drive. Any clues?
Usually a sign of a bad hard drive cable. Consider getting that replaced first. Try installing Mac OS Mojave via the USB install media method with the SSD installed in the internal drive bay and see how that goes. Problem with creating a bootable installer is that it requires a working mac to create the installer, I assume you already have the installer as you’ve installed new MacOS recently. MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable Here are instructions to do it if needed for usb install media: How to create a bootable installer for macOS
