Installed that driver (from: http://www.5v.ru/ch340g.htm) and had a port configured.
http://www.wch.cn/downloads.php?name=pro&proid=178.http://www.cindori.org/enabling-trim-on-os-x-yosemite/Just to summarise. On Yosemite:1. install the driver2. Run this:Code: [Select]sudo nvram boot-args="kext-dev-mode=1"3. Restart
sudo nvram boot-args="kext-dev-mode=1"
Actually, I was too quick assigning blame. It was nothing to do with the USB-serial at all, and everything to do with the capacitor on the Arduino power supply. (presumably a large current surge when plugging-in caused a rather serious problem. But it shouldn't crash the Mac surely?)