Upgraded to SSD and Now the Loading Bar Does Not Complete

Your facing a common issue! The original HD this system has was a SATA II (3.0 Gb/s) drive, the HD SATA cable was likewise only rated for SATA II. The system its self with the newer firmware back in El Capitan and onwards reset the SATA controller to SATA III (6.0 Gb/s) which is all great for your new SSD which is likely SATA III (6.0 Gb/s). But the cable is the weak link as its not able to support the heavier SATA III data flows!

OK, so I now understand why my system is not working how do I fix it?

You’ll need to replace the cable here’s the needed part MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable and here’s the guide you’ll need to follow MacBook Pro 13" Unibody 中期2012 硬盘线缆替换

But even still you could also need to do a few more things! First you want to apply a stripe of electricians tape on the Uppercase where the cable crosses over to help protect it from abrasion from the rough aluminum surface. In addition you need to be careful how you form the bends, You don’t want to fold it as the crease will damage the cable you want a nice arc. To help with the radius I use the ink straw of a Bic ball point pen or you can use a bamboo cooking skewer. Once done I do recommend you reformat the drive and install a fresh OS like you did before.