One more thing, you may need to enable TRIM once you get your SSD installed.
You do it in terminal if you need to, but it is just one line entry. You can search it up or ask here for help if you need to enable it, but it is a really easy thing to do. You check to see if you need to enable TRIM in the system profile, it is under the SATA section.
Also, if you need help or advice on anything else with the SSD install, just ask us here. But, like I said, that particular upgrade is really easy, and I wish it was that easy on other Macs.
When I added a SSD and RAM to my Late 2011 17" MBP, it took maybe 10 minutes max for the physical install.
I have a suggestion for SATA/USB cable:
Product Description:Take advantage of the lightening speed of USB 3.0 to quickly transfer files up to 5 Gbps! Connect any 2.5'' Serial ATA (SATA) / SSD or SATA II Hard Drive to your computer through an available USB 3.0 port on your desktop or laptop. Reverse compatible with USB 2.0 and 1.1, this...
www.amazon.com
I bought a few of these for my 2.5" drives. I like them over using enclosures because they are quick in speed and assembly, have long cables, and cheap.
This could be an issue as it is not uncommon, but again, I would bet money that it is the HDD. I personally like to keep things simple and replace the stuff that needs to be replaced. At least on something like the Mid 2012 MBP.
If this was a more complex install or a Mac that was more difficult to open, then I would consider doing the SSD, RAM, and SATA cable all together, but it only takes a minute to open the Mid 2012 MBP. I would just replace what you know matters, and leave the other stuff for later if they are a problem as well.
One potential issue with doing multiple things is that if you get your MBP all back together and stuff isn't working, you don't know which one of the things you replaced is causing the issue.
Of course, if you would just want to knock it all out together, thats okay too.