OK I think I know what has happened now.
I have a range of drive devices on my PC and I think the limit of drive letters is E: because the CF card reader has appeared as E: but my previous D: hard drive has disappeared.
I think this happens because I have a home network and the F: drive may be the first network drive.
Now all I need now (I think) is to find out how to make the first network drive G:.
Anyone got any other thoughts