I have a WD Elements 1.5 TB External Hard Drive. When I tried to drag and drop files onto it I got a message that said that the disk could not be modified. So I tried reformatting it with Disk Utility, Erase. I selected Mac OS Extended as the format, and clicked Erase. A message came up that said: "Disk Erase failed with the error: File system formatter failed." The disk icon disappeared from the desktop. I tried again using MS-DOS (FAT), and it worked, and the disk icon returned on the desktop. I can now drag files onto the drive. Is there a reason I can't use a Mac format? Can I just go ahead and use the MS-DOS (FAT) format? Is it compatible with OSX?