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". 

To Resolve:Edit

1. 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} - chkdsk /x

2. If it starts returning a bunch of errors, upload the Western Digital Lifegaurd Diagnostics Tool and do a "Quick Test" or "Write Zeroes". Any errors here is immediate RMA. Take note of the serial number of the drive prior to calling the manufacturer.