delayed - failed back up
Failed message+ the backup volume could not be mounted. What does this mean?
It probably means that it needs to be formatted first. If you are running a Mac go to Applications -> Utilities - Disk Utility and select the drive on the left and format it. What computer and what operating system are you using? How is the drive connected?
There could be a couple causes for this:
Or like mayer said, the drive may need to be formatted. If you already have data on this, you should definitely check the above ideas first so as to not lose vital data.
I always verify my disk in Disk Utility every couple months to make sure that the disk isn't corrupted or failing. My time machine disk corrupted once on me, and I lost all of my backups (I found out when my time machine disk failed to back up my hard drive one day).