Difficulty uploading a modification to an existing sketch

I have been using my nano for reading 6 voltages and sending the results to a sd card. I am having trouble getting the IDE recognising the usb port. The select port is greyed out. Usually removing the cable and changing the board clears the problem but even reinstalling the IDE does not clear the problem. I have used the same port for other usb devices without problem. I am running Ubuntu as a OS. Any help will be appreciated.

do you have anything connected to the nano when uploading, especially to the hardware serial? might interfere...

disconnect all and retry ..

Thanks for the suggestion. I have a sd card reader with a card in it. I will try disconnecting the card reader.