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I am running VB 5.0.8 on a MAC with El Capitain. VB is running Windows 7 Premium 64 Bit. I am able to install the Arduino IDE version 1.05 (Need to run this version because of application drivers fail on newer versions of Arduino IDE.

When I plug in a Arduino ATMEGA2560 Windows sees the device and adds it as COM port 3. The IDE see it but after compile I can not upload to the device. I get AVR timeouts. In native MAC all is ok.

For the purpose of this discussion, the Arduino is essentually a USB-RS232 adapter. Perhaps it would work better if you let the host look after the USB half of this connection, and map the host serial port to the guest (as opposed to mapping the USB). RS232 doesn't have any built in timing issues.

I could try that but I am not familiar with the command to do that on an Apple iMac. Can you give an appliance operator the instructions?

I think it is more likely a Virtual Box issue with USB support under El Capitan. Parallels 11 works fine.

Sorry, no, I'm not a Mac user, so I can't give you detailed instructions. Perhaps Google can help. All you really need to know is (a) does OS X even support serial ports?, (b) the host device name if so.