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I am trying to use an FTDI breakout board to connect to my custom made Arduino compatible board. I am using a new 16 inch macbook pro with USB-c ports, so I am using a dongle, in case that could be the problem.

Every time I try and upload a sketch I get the error: "can't open device "/dev/cu.usbserial-D306P6WW": Resource busy". When I check the /dev/ folder, both "/dev/cu.usbserial-D306P6WW" and "/dev/tty.usbserial-D306P6WW" show up, however in Arduino IDE I can only choose the "/dev/cu.usbserial-D306P6WW" option. I have installed FTDI VCP drivers, CH340 drivers and all kinds of other drivers to try and fix this issue. I have restarted countless times, and I have used a multitude of commands to check for programs hogging the usb port, but there is never a program. Plugging a harddrive into the same port causes no problems, so the port can't really be the problem. I am using a "FTDI FT231XS" board from Sparkfun.

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    Do you have the Arduino serial monitor opened in parallel? Also, from where do you have the FTDI drivers? Have you unloaded the mac os kernel drivers before that? – Maximilian Gerhardt Feb 8 '20 at 15:36
  • I don't have the serial monitor open. I downloaded the FTDI drivers from their website, and I have not unloaded any drivers? Do I need to do that? – Thijs van der Heijden Feb 8 '20 at 15:38
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    Hm actually I confused it with the CH340 installation process (github.com/adrianmihalko/ch340g-ch34g-ch34x-mac-os-x-driver). But that shouldn't be used for such a recent Mac OS version; also you don't need it because you don't have a CH340 chipset. If you followed learn.sparkfun.com/ftdiDriversMac you should be good. Can you open a root terminal and edit your question to include the output of kextstat | grep FTDI? Unplug and replug the FTDI board and post the recent dmesg output starting at the unplugging of the USB device. – Maximilian Gerhardt Feb 8 '20 at 15:44
  • Thanks for your help, I tried a whole bunch of other things, and then decided I would update my Mac version. For some mystical reason this actually fixed the problem?! Thanks anyway! – Thijs van der Heijden Feb 8 '20 at 16:43
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Nevermind, updating my MacOS version to 10.15.3 magically fixed the problem...

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