Luiz has a hard drive that Windows can't read and it wants to reformat it. Leo says chances are that it was formatted with Apple's file system, rather than Windows file system. Leo recommends copying the files off of it using a Mac and then reformat the hard drive to FAT. Then copy it back. The Windows machine will be able to read it then.
Another option is to just buy a second hard drive that is already preformatted in FAT and then just copy them over from the other hard drive with the Mac. Then he'll have two copies, one being a backup. It'll then be a simple drag and drop.