Here is your basic problem OP:
It is just a BAD idea to put your XBMC install on your media drive.
Why?
Because they need different things. Your media drive is best off being one of the above drives (aka a Samsung or WD green drive) as they are more than enough to playback even the meanest HD files but they run cool (compared to 7200 RPM drives). Hot drives over time die.
But if you install and run XBMC from these drives the experience will be far more sluggish (because of the green drives) than if XBMC ran from a 7200 RPM drive or a SSD.
I have said it once and I will say it again: Anything less than a dedicated HTPC hooked by ethernet to a dedicated server is just a stopgap. That is why this is not a problem for many of us: we put the green drives in the server and NO real drives in our acutual HTPC.
But it is understandable that you don't have the budget to do that now- we were all there at one point. But it is still smart if you prepare for that future and separate your OS install from the rest.
My suggestion?
Run the OS off a pen drive, and just get the large green drive for storage (yes, a green drive), external or internal.
Then when you can afford a SSD later you can replace the pen drive, and when you can afford a NAS/Server later you can migrate the storage drive to that server.
Whatever you do, do NOT install the OS on a Green drive and DO NOT get a large 7200RPM drive to overcome that. Just please give up having the media and the OS on the same thing- it will save you SO much pain in the long run.
Pen drives are pretty cheap nowadays...
Last note: I personally prefer WD drives, but neither of the above two are "on sale now." I got the 2TB green for $109 shipped a week ago and in the last month I have seen the Samsung go for $115 shipped. Since your budget is tight buy the HD last, and maybe you will get lucky and hit a sale by then..
One last last note: Lets say you throw away all the advice in this thread about separating the OS drive and the data drive. You say to yourself "I only care about low definition video so I can get by permanently on 2TB and I don't want the extra expense of a pen drive." Then get one of these drives- they are the only 2TB drives that would have decent enough performance to install XBMC on (their price is a little high now):
http://www.newegg.com/Product/Product.as...6822145298