Thanks for the pointer, but the problem was a bit more fundamental. After some investigation I discovered that having only reformatted the C drive not the D, I no longer had the correct access powers, I changed these and I could see the files. As regards the external drive I think I must have unplugged it while it was working so messing up the file index. My son's Mac could read it though so I copied off the relevant files and reformatted the disk. All is good now thanks PudgyChicken!