I have isolated the problem somewhat. It only occurs when writing to a drive that is plugged in with firewire. Writing to an USB-plugged drive is fine, albeit slower.
I've had a B&W G3/350 Yosemite for ages, with two Argosy enclosures hooked up by FireWire. They've worked flawlessly.
However, when I got a Mac Mini and hooked them up to it, strange problems show up. Seemingly related to disk activity, a drive will lock up and require for it to be rebooted...