As one senior to another, yes SATA or eSATA is the way to go, but does your motherboard support that?
Can you tell us the specs of your PC? That would give an inkling of the best approach in your case.
What is the brand AND model of your motherboard?
What does the motherboard manual say about the number AND type of hard drive connections?
What is the size, in Watts, of your power supply?
What is the brand AND model of your computer case?
What are the number AND sizes of your computer case fans?
Does your computer case have any UN-used case fan spaces?
That is a lot of homework. I will get to it and get back A.S.A.P. It, the computer is not a brand, it was built out of parts. The gentleman who built it is no longer availabe to me.
The system process in control panel MAY give you some of that information
It should ALL be available with the information the system builder gave you... he DID give you the paperwork that came with the individual components?
Taking the side panel off the case and looking inside with a flashlight might also help... but, of course, you'll need to know what it is you are seeing, and it sounds like you don't know much about hardware, so that may not help much
Sadly, if the person who built this "video editing" computer didn't really understand video editing, you are left with a computer that won't do what you want, and no easy way to make it work
You may need to find a computer shop to do this for you... and make sure part of their price is giving you a written list of everything you HAVE and everything they DO in the way of additions or configuration changes