External FW HD on Mac Mini suddenly broken


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 (resulting in losing all the data that wasn't flushed from the cache).

I've tried using the drives both as USB2.0 and FW, and tried different cables with the same result.

Disk Utility reports no problems with the directory structure on either drive.

Basically the only thing about the setup that has changed is that where I was running 10.3 on a Yosemite before, I now run 10.4 on a Mini.

Any help would be appreciated! Thanks in advance
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.