All my older UNO's with Atmega 328P chip load and large USB cable load OK My new UNO R3's with mini USB socket and same Atmega 328P chip will not find a COM port. The L bootloader light flashes OK. Any clues to fix would be appreciated. Please note I am not an absolute computer expert but can find my way round if guided. Jet
Try using a different known-good USB cable or verify that the USB cable you're using works for data on a different device. You might have a charge-only or defective cable that only provides power but doesn't have the data lines connected.
I think you are absolutely right! Didn't think of that as I tried 2 different cables to no effect. However, after reading your note I've just found another one that connects my phone in my car to hands free and now the UNO does find a port. However I now get a 'compiling' error on unmodified software that loaded ok before. At least I'm one step forward. Thanks very much.
Glad to hear you have the port at least. If you want help with the error: When you encounter an error you'll see a button on the right side of the orange bar "Copy error messages". Click that button. Paste the error in a message here using code tags.