Test your external hard drive on a different Mac or pc sometimes, this problem occurs due to the hard drive damaged.
I evaluated some possible cusses that are going to mention below. If you connected your external hard drive with your Mac, and it's not responding like discussed above, definitely there is a kind of particular problem behind the scene, that you didn't recognize yet. Why does my external hard drive keep disconnecting? Change the format of your external hard drive Recover or backup your drive data before format Check if your Mac can recognize the external hard drive
