hi! i just bought a toshiba 320gb canvio portable external hard drive that i want to store video files from my macbook pro on. i connected it by usb to the macbook pro and it came up on the desktop and i tried to drag some video files onto the icon for the hard drive but it wouldn't let me. on the box it says that for the external hard drive to work on macs it needs to be reformatted. what does this mean and what do i need to do to make this thing work? thanks in advance!