Hey, tech, here's some help with your hard drive problem.
It looks pretty easy. You just undo a couple screws, pull out the hard drive in its casing, and undo some screws on the casing and pop the hard drive right out.
Then all you have to do is find a 2.5" hard drive and replace it.
Here's one of your cheaper options.
Personally, I'd pick this one (out of these three.) 500GB is a nice size, and 7200 rpm is nice and fast for a hard drive.
You could always go for this one too, but personally, I wouldn't. At this time, I don't really trust small TB hard drives. I've heard about overheating and you're cramming so much data on to the disc that there's a lot more room for error. If you REALLY want a TB hard drive, I'd recommend just buying one of the full, 3.5" external hard drives.
I hope this helps with you hard drive problem!