Installing 2 new SSDs

You’ll need to decide which drive will be your boot drive. With it located where your HDD was you’ll set it up as the Boot drive. But we are still a few steps ahead here!

First you’ll need to replace your HD SATA cable as its likely the older version which can’t support SATA III (6.0 Gb/s) data flows and/or its worn! Your Hard Drive Cable Is A Ticking Time Bomb. Here’s the cable you’ll need MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable and here’s the guide to put it in MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement. So place a strip of electricians tape as outlined above. But we do need to be careful in handling the cable as you can easily damage it! You don’t want to crease it, instead you want to use an old BIC pen ink straw as a bending brake to form a nice arc not exceeding the radius of the straw where you need to bend the cable around a corner.

OK after getting the boot drive installed you’ll need to then install the OS onto your boot drive. I’m suspecting your girlfriends Mac was running Sierra or older macOS which uses HFS+ unlike High Sierra and newer! Which uses APFS. During the OS install process to this systems drive the installer will update the systems firmware and upgrade the file system on the current drive.

But I really don’t recommend going to High Sierra or the newer Catalina release. This is where you need to remember older 32bit apps won’t work under Catalina and as this system is SATA based it really works better under HFS+! APFS is really ideal with PCIe/NVMe drives as they offer deeper buffering with its more chatty dialog.

In any case I would recommend you download a fresh OS installer image from here: How to get old versions of macOS. Then follow this guide to create a bootable USB thumb drive installer How to create a bootable macOS Catalina installer drive or How to create a bootable macOS Sierra installer drive

Then install your second drive prep it up with Disk Utility as your data drive.