Chosen Solution
I have been trying to install a new 240gb Sandisk SSD Plus into my mid 2012 macbook pro but keep having issues on the reboot. I go through the necessary steps of installing the new drive into the computer and copy the files from the old drive when it is externally connected. After, I install the operating system and the computer will restart after install but opens to a blank screen. If i type in the login password while at this blank screen it will bring me to the desktop while running on the new SSD. However, once I try to update the system and has to restart it gets stuck on the loading screen with the spinning wheel below the apple logo. I have done the recovery startup which allows me to get to the disk utility and have made sure the 1 partition is selected along with the (journaled) operating system then reinstalling the operating system but still has the same issue. I have also tried to use the old hard drive externally connected to reboot but the same wheel appears and wont turn on. When I originally tried installing the new SSD I first had it externally connected and the old drive internally connected but was unable to make any changes to the disk. If i tried to start up to the externally connected disk it would not recognise it and only would have the old interally connected drive as an option. I am not sure what else to try so any help would be appreciated!
If you didn’t already you may want to try replacing the hard drive cable as well. I recently performed the same upgrade & had similar issues, but they completely disappeared once I replaced the HD cable. It also sped up my machine quite a bit. (I realise that you probably do not have this issue anymore, but just in case it pops up again, and/or for anyone else who may have it.)
