Performance Alert Message

I have Noton Internet Security 2012

 

I am attempting to back up a PC to an External Drive - all works well up till about 57% when Windows is doing a System Image. Shortly after commencing this procedure,  the following message pops up:

 

 

"High Disk Read, Disk Write usage by: Microsoft Block Level Backup Engine Service EXE"

 

and within about two minutes, the back up stops and the message is 'Back Up Unsuccessful'.

 

I have confirmed that the External Disk has no probs as well as the internal hard drive. I have also been able to back up another PC  (also  running Windows 7) using the same External Drive, without any probs.

 

Any clues as to how I can eliminate this problem during back up, would great be appreciated.