MacMini not booting when Uno R3 is connected (and sending Serial Data)

Hi everyone, i got a problem with some Uno R3s. I have an 12Bit ADC connected over SPI and sending the Data Serial to a MacMini. Everything works as expected. Now when for some reason i have to restart the Mini it freezes on Startup Screen (the gray one, just before the Apple appears). If i try again without the Uno connected it starts up as usual.

Interesting: If i use an R1 Uno it also works!

It also is the same with six different Arduinos (R3)

Does anyone got similiar experiences or (even better) an idea how to fix that?

Greetings and thank you in advance!

Jewe

Disconnect the arduino when you boot. Not knowing exactly what is being sent over the serial connection, but you could be overflowing a serial buffer, or the Mac is interpreting the data in some way that confuses it into thinking the arduino is some sort of boot device.

Thanks for your reply. I was hoping that i wouldn't need to disconnect the Arduinos as all seven of them are build into an installation which is not perfectly accessible during the show. Well i'm kind of hoping that i won't have to restart the Minis at all.

Anyhow in the meanwhile i found out that (for some reason) it seems to work if the Arduino is connected via an active USB Hub. I'll keep an eye on that.