I placed old hard drive in “optiBay” enclosure and Samsung 840 EVO 120GB instead of main hard drive.
I had data backup done earlier, so I opted in for complete wipe/format - booted into recovery mode - installed fresh OS, I’m keeping my OS and apps on the SSD and home folder on HDD. It boots up very fast, apps are starting up like crazy
I had huge problems with heating up .. I even got a “leg hair burns” :)
I followed this instruction, replaced thermal paste (nothing fancy .. just some “normal” paste from local IT store), cleaned out all the dust (especially from fan)
now its running like new … fan rarely kicks in and when it does it cools it down really quickly. Of course running some intense apps (like lightroom, starting up photos app, many open chrome tabs with youtube etc … ) will do a lot of stress on CPU so it will heat up.
In general some slight “heating up” is always there .. Apple prefers to run their laptops warmer than spinning up fans. But after proper cleaning and replacing thermal paste it is much more manageable.
Small tip: those really small ribbon cables can be a pain in the a** to put back in, so i got them close to the connector, put a little sticky tape on them and used that sticky tape to pull on the cable and push it in the slot.