Thanks everyone for your comments.
Earlier today I tried to back up to a different external HD, but got the same results i.e. backed up about 6 GB and then aborted with 11 errors!!!!
I then tried again, but this time upon opening SilverKeeper it advised me that I had to "authenticate", which was just two OK's, and lo and behold the complete back up was completed.
I tried it once more on the WD HD, just to double check, and again the back up was carried out without a hitch.
I'm none the wiser what caused the glitch, but thanks again for your time.