I have always found the backup to an external USB disk drive hit and miss. Basically in the middle of a backup, Windows 7 (64 bit) seems to unmount and mount the disk drive, causing a backup failure.
Having installed the latest version of N360, this problem has been substantially reduced - I can now do about 80% of a backup before it happens (under the 2013 version, it kept happening at around 10% backup ) - which does suggest it is something to do with N360 rather than Windows...
However, I was running my backup today and at around 80% I just got the message 'Backup Failed' - More Info opens a new window, which is blank - see screenshot
Not a very helpful screen is it.... - even another click to More Info....