Thank you 'mende1'
I tried to follow that which was explained, but when I click on 'Repair Disk', within moments I get a red warning that the disk cannot be unmounted.
However, if I click on 'Verify Disk' a check is made just as I would expect and reports that all is well!
I'll spend some time today disconnecting and reconnecting the disk (it's a new one!) Might there possibly be a firewire connecting cable problem, I wonder!