thank you for replying. it is code 198 Uncorrectable Sector Count. it now stands at 1, but the smart status is saying this is OK. before it registered this Uncorrectable Sector it was reading code 197 Current Pending Sector Count and that was at 1, i think.
ive zero filled both drives, one with killdisk and one with hdtune. and there both reporting 0 bad sectors.