Do you get any more information (another error message) other than "Backup failed"? Make sure you can access the drive in windows explorer and that the HDD is not write protected. Also, an anti-virus software you are running on your computer might be accessing certain files that are used during the backup process. Stop or close any anti-virus programs and then try the back up again.
Here's the procedure for checking the disk status.
Open cmd window running as administrator
Run diskpart.exe
At the "DISKPART>" prompt enter the following commands. Text from // onwards is a descriptive comment
>list volume //shows volumes and drive letters
>select volume n //set focus on desired volume number n from list
>detail volume //gives detailed attribute list for selected volume