So i bought this new 500GB external HD but whenever i tried to put my files in it from my mac it wont allow me and say, "This item could not be moved because [harddrive] cannot be modified." however if i connect it to my windows it works fine. i've searched online but i dont know what partitioning exactly mean so i came here for help. thanks! P.S just one stupid question, why would a 500GB HD has less capacity such as 470 or so; and sometimes 8GB USB will only have 7.5 G etc.