This error can be related to several possibilities:
Incorrect credentials for accessing the device
This is generally related to network resources or Unix based NAS devices. As a work around for this issue, what you can do is create a new local admininstrator user on the local machine performing the backup. Then create a user on the NAS device with the SAME user name and SAME password that was created on the local machine. Set the job to run as that user. This should bypass any type of authentication issues between the two devices.
Missing / sleeping usb drive for the backup location
Make sure you disable any power saving settings by going into the device manager, and changing the properties for the Root USB Hubs on the computer.
Lack of available space on the backup location.
Make sure there is enough drive space to backup your data. A good rule of thumb is to use n*3, where n is the amount of data being backed up. So if you are backing up 50GB of data, you should make sure you have 150GB of free space on the drive.
Check that you are using the most recent build, as we do not see this very often with verison 11.