Your 500 gb external western digital hard drive is corrupted so that it prompts to format, now if you need your data then you have to go for a data recovery software, i will suggest you to use "Kernel for FAT and NTFS" hard disk data recovery software for the recovery of your stuff data and files from your hard drive. It is an efficient data recovery software which has a user friendly interface and has a unique feature that it preview the lost data in RED color in scanning of your drive, it gave you the choice to save all at once or some selective files to your location.