You’re external drive is not recognizing or you have multiple “Disk-Error” messages in the eventvwr.msc. Backup programs may report the backup failed due to “corruption”.
run => cmd => (Drive letter of the external) => Press Enter to move to the next line so that you can run the next command from the drive directly =>
If it starts returning a bunch of errors, upload the Western Digital Lifeguard Diagnostics Tool and do a Quick Test. Any errors here is immediate RMA. Take note of the serial number of the drive prior to calling the manufacturer.