trying to reset factory settings before upgrading but itunes says error has occurred when backing up

Geeting iphone reset to factory before new 5S arrives next week (hopefully)

tries to reset the factory settings and do a full back up to my imac before resetting but message comes up saying an error has cccured "itunes could not backup iphone because an error has occurred" and stops

I tried to back up ot imac and didnt realise last back up to imac was in january, how do i get past this to backup and reset my iphone

Can anyone help

Thanks

Ewan