It is incredibly annoying when you are transferring data between two devices. Whenever these kinds of problems occur, your external hard drive doesn't show on your Mac.

Test your external hard drive on a different Mac or pc sometimes, this problem occurs due to the hard drive damaged.This problem might because of your mac or USB port( thunder-bolt), so try a different Mac to assurance.If your external drive connected through a wall outlet, use a different wall outlet.A USB port( Thunderbolt port) is damaged, it might be a hardware problem of your Mac, so try a different port.Data cable is damaged or exposed( try different cable).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

Why does my external hard drive keep disconnecting?.

Backup my mac on external hard drive