Upgrade from Mountain Lion OSX 10.8.5

You raise a few different directions here lets see if we can put this in a more logical order…

If your current HDD is getting full or is slow then you could clean off the junk making space and you could also run a defragmenter to speed up your drive. Over time HDD’s get frag’ed this is where a given file is broken up in to many parts so it slows to loading and saving of the file. This will take some time and you do want to make a full backup before you start this. The other direction is just replace the drive and then Migrate your stuff from the old drive to the new drive. As for what drive to get you could go with a SSHD hybrid drive which gives you the zing from the SSD cache and yet the deeper storage of a traditional HDD. Or, you could go with a SSD. But! do keep in mind you need to leave 1/3 to 1/4 of the drive free depending on the size (the smaller SSD’s need more free space).

You do want to replace the HD SATA cable when you replace the drive. The original cable is not rated for the higher I/O flows from either the SSHD or SSD MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable I also place a strip of electricians tape on the uppercase where the cable crosses over to help protect it and you don’t want to bend the cable sharply as that can damage it, you want a smooth arcs.

The last action is to max out the RAM to 16 GB if you can. More RAM will help your system deal with large files and applications.

OK, thats the hardware side, now lets talk about upgrading the OS.

I strongly recommend you only go to Sierra 10.12.x. High Serra and Mojave introduce a newer file system which does not work well on SATA based SSHD or SSD drives. As for getting the OS If you’ve accessed it from the Apps Store you can get it again, If not you can go to Apples downloads and heres a bit more on how to install it https://www.macworld.co.uk/how-to/mac-so...