Thanks, Gary, for your suggestions.
I am using ubuntu which has been updated to the latest edition which works well. The HD is a 20gb drive on which I installed ubuntu about one year ago but had not used for some time There is 512mb RAM installed.
The strange thing is that neither the bios nor ubuntu recognise the HD but do find a 4 gb usb stick.
I can live with this but was just wondered why this happening.