AND_YOU_ARE and mrpete. Thank you both for your replies. I unplugged the usb plug and tried another usb socket. And it worked o.k.. Looking in Device Manager, the error had disappeared and I've been able to save an incremental image to the external drive so it seems to be working again. Also I checked the usb socket which used to serve the external drive by inserting a pen drive, which was also recognised. Haven't a clue what went wrong but it all seems o.k. now. Again many thanks for your help.