Your external drive is most likely corrupted and needs to be replaced
Open cmd
Type in
Chkdsk /f (drive letter that is your external drive. Example. Chkdsk /f d:
Afterwards,
Open disk management (with your backup drive still plugged in)
Locate your external drive in there
If it is listed as RAW format, it has been damaged.
Check if it is failing first with a smart test: http
/www.howtogeek.com/134735/how-to-see-if-your-hard-drive-is-dying/
Contact the company for that backup drive, they probably will replace it for free