Norton 360 backup to external drive

Hi - I have been using automatic backup on Norton 360 to an external hard drive (drive L) for a few years now with no problems. It usually backs up when the system is idle. In the last few days I have had an 'At Risk' message telling me it hasn't backed up recently, but when I try to run backup I get an error "There is a backup problem. Make sure the destination drive is connected and then try again".

 

The L drive is showing up and reading fine from 'my computer', the drive is 1.5T and is only half full and it is also showing in my Norton backup settings as the L drive. 

 

So why won't it back up?

 

Any suggestions greatly appreciated.

 

Regards

 

Terry