First, although I doubt this is the issue, it could be a problem with your USB setup and could fixed by simply uninstalling the USB device that's failing and then reinstalling it. See the link below for instructions.

https://www.pcworld.com/article/2902263/this-trick-can-quickly-fix-many-mysterious-hardware-failures.html

And second, and this is the most frequent reason for such errors on external hard drives, the hard drive has failed or is failing. External drives are famous for dying at inopportune times and that's why we recommend ALWAYS making a backup copy of the external drive so you actually have two identical setups. I'd start by plugging the external drive into a different computer and see if the other computer is able to recognize the device. If so, copy ALL of the files from the external drive and make a backup copy of the files. If that doesn't get it done, then you might purchase a new external enclosure and swap the hard drive into the new enclosure to see if allows the drive to be recognized. (For those that have an external enclosure laying around, this is a cheap way to check the drive but if you have to buy a new one, you may decide on another course of action.)

If this doesn't work, then the drive is probably gone. Hopefully you previously made a backup of all the files on your external drive because you'll need the backups to copy all the files to a new external drive. Of course, if the files are extremely valuable, there are recovery companies that might be able to recover the files for you but it's expensive.

Hope this helps.

Grif