Access denied when Uploading I went to upload my sketch to my Uno (r3), but unusually, the Upload Console returned this error:

avrdude: ser_open(): can't open device "\\.\COM4": Access is denied.


Just started seeing this, cannot upload.
There might be another device has already connected to that port. It could even be a terminal emulator which is left connected from last debugging session.

Otherwise, make sure that your UNO is really presented on COM4 by checking with "Device Manager". if not, change the port where it is available.