Each time I attempt to upload to the MKR WiFi 1010 board the COM-port gets disconnected, usually at one of the following points:

  1. Either it disconnects just after compiling/verifying. In this case the LED_BUILTIN pulsates slowly, and the USB-cable needs to be reconnected in order to try again. The next attempt usually results in 2..

  2. Or, it disconnects after a successful upload. The next attempt to upload will always result in 1. occurring. When this happens the COM-port is still visible in Windows Device Manager but it can't be found by a Serial Monitor.

Any ideas what could be causing this or what could be done to have it stay connected? With an UNO this doesn't happen.

  • does it happen only with your sketch or with Blink too? Uno has external Serial chip. on MKR the MCU handles the USB so as it resets the port disconnects
    – Juraj
    Commented Apr 27, 2021 at 16:29

1 Answer 1


This was due to using a virtual machine (VMware) which apparently doesn't handle the MKR board COM-port so well. Moving the development environment to the host solved the problem. It still disconnects the COM-port after upload but then consistently reconnects to the same COM-port and that's good enough.

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