Flashing using external UART adapter with a board that has a USB controller

[ooddxx]

Hello,

I struggle with flashing a dev board that also has a onboard USB to TTL converter directly using the RX/TX pins...


On the console through the external adapter I can see:
[Codebox]
rst:0x1 (POWERON_RESET),boot:0x2 (DOWNLOAD_BOOT(UART0/UART1/SDIO_REI_FEO_V2))
waiting for download
[/Codebox]

Where the flashing command is stuck at
[Codebox]
Connecting........_____....._____....._____....._____....._____....._____....._____

A fatal error occurred: Failed to connect to ESP32: Timed out waiting for packet header
[/Codebox]

I tried holding down the BOOT Button all time without success.

Flashing through the onboard USB controller works just fine.

I am trying to engineer the circuit I want to print to a PCB so I want to test as much as possible to avoid mistakes in the print...

Is it possible at all to flash with an additional adapter or will they conflict as they are both connected in parallel?

Best