Hello, I am running out of space on my 250Gig hard drive on my hp pavilion dv7-1135nr. I've seen a few posts stating that I can add a second hard drive, but the space for it looks to be filled with something else. If I can't add a second hard drive, I'd like to replace the current one with one that is at least 500 gig. Can I just buy any SATA drive, or are there different sizes and compatibility problems?