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Board won't show on one of my computers

I have the same problem with these two boards: Ruggeduino (Uno compatible) and DFRobot Romeo 1 (Duemilanove compatible). On one of my Macs (a Macbook Pro running 10.11.4), I can see a new port /dev/cu.usbmodem641 or /dev/a4005g3t depending on the board. On the other computer (a Mac Mini late 2014), they just won't show. Same cable, same boards, and no hub. I don't know where to look to see an indication of a failure of some sort.

These two ones use a type-b USB socket (printer style)

Also on the Mac Mini that won't see any of those boards, I can connect an EtherMega2560 (Arduino Mega 2560) which happens to use a micro USB socket. I don't know if that is related to the problem.

Both computers run the same OS (OS X 10.11.4) and the latest version of Arduino.

Here is a difference (found in /var/log/system.log):

On the working computer:

Mar  3 22:20:42 pc101 kernel[0]: AppleUSBFTDI: fInBufPool,kMaxInBufPool 8,64 
Mar  3 22:20:42 pc101 kernel[0]: AppleUSBFTDI: Version number - 5.0.0, Input buffers 8, Output buffers 16

On the non working one:

cfprefsd[114]: BUG in libdispatch: 15E49a - 1718 - 0x0