go on ebay and pick up a case for a standard size hard drive for 15 bucks and go to newegg and get a maxtor sata 3.0 500gig for 175. 500 gigs of externalness for 200ish sounds good to me
I've experienced that internal harddrives in harddrive cages can give major hardware issues, from pc's not recognizing the drive to the drive not even spinning...
I have an external 500gb Seagate that I got for around $200 - $250. If I could have gotten a different one, I would have gotten a Western Digital My Book. From what I heard, it turns on with your computer automatically.