The SATA cable is the weak link, one of many reasons Apple went to the PCI-e and soldered-in storage.
They can be damaged quite easily if the unit is mishandled during assembly or re-assembly. This type of cable can experience cracks in traces due to aging, heat, vibration, impact and abrasion. All OWC SSDs backed by a lifetime warranty. Those SATA cables are actually not cables but flexible circuit boards, usually mylar, with printed circuit traces taking the place of wires. Get everything you need to upgrade your 2012 MacBook Pro SSD for a total system performance and capacity boost. As it cools down, electrical contact may once again be restored. As a unit heats up, it expands, and as it expands, the crack widens, eventually forcing a complete electrical separation to exist on both sides of the crack, hence a failure. In the case of an intermittent generic failure, the electrical continuity of a cracked trace on both sides of the crack is very often a function of the temperature of the unit at the point where the crack exists.
It is however no mystery why old cables fail when switching from HDD to SSD. Agreed, a new SATA cable is a nominal expense and highly recommended.
