ESP8266 Community Forum

It is true the D1 mini like other similar boards will upload / program automatically via the USB from a suitable IDE but this assumes that nothing extra is attached to the critical GPIO pins during this operation.

GPIO15 is pulled low on the board and GPIO0 and 2 are pulled high on the board. The upload sequence automatically pulls GPIO0 low to put it in serial flash mode. However, if any of these 3 pins is externally coonected to other logic signals that could disturb the process. It sounds like GPIO2 may have been pulled low by something external.

Thanks for your answers.
My D1 was always unconnected (so only USB connected) when trying to flash (always with Arduino IDE)...
I also tried again after uploading an empty sketch...but same result, only works when pulling the right pins down/up.
No idea why it doesn't work automatically (after the very first flash which wrked automatically).

But it's ok now, I know how I can flash and it works reliable like this.

Thanks! That did the trick.

D3-Gnd
D4-3.3v

I had the same problem. My guess is also that serial port is too busy and connecting for flashing is frustrated by that.