NOTE: We have updated the NANO board with a fresh bootloader. Boards sold from us from January 2018 have this new bootloader, while boards manufactured before that date have the old bootloader. First, make sure you have the Arduino AVR Core 1.16.21 or later looking at the Board Manager. Then, to program the NEW Arduino NANO boards you need to chose Processor > "ATmega328P". To program old boards you need to choose Processor > "ATmega328P (Old Bootloader)". If you get an error while uploading or you are not sure which bootloader you have, try each type of processor 328P until your board gets properly programmed.
Of course the answer to this Nano problem is hidden away in 12,500 words of Arduino Software Release Notes. But who would think of looking there?This is one of the annoying aspects of the Arduino. Although it is great fun, it is so 1960's - when weekly updates used to arrive highlighting the effect of changes that had been made weeks or months before.Somewhere someone must have known that this update was going to affect people with Nanos, but without speedy access to known solutions a whole crowd of people (looks like more than 26,000!!!) have been floundering around. I rest my case.
I think there is a 'not' missing from your final sentence ;-)
wasn't it a new IDE that caught people out?
Your Troubleshooting page is certainly a good idea and I would like to contribute to it
see it transferred to this forum.
Maybe our last few posts deserve a new topic to attract a wider audience?