its cheaper to get a harddisk and then a casing rather than to get it as a set from stores...
the 3.5" 300gb seagate costs ard 220 if i dun remember wrongly...and a casing would normally set u back 50-80 bucks? most prob it will be usb but 3.5" needs power supply.... though a 2.5" doesnt need the power supply...juz usb...but it will be more ex....hahahah