This probably doesn't help many people, but if just one kid is saved...
Your FireWire interface card has to have either a LSI or Texas Instruments chipset on it, or your MOTU device won't work (properly, if at all). MOTU obviously don't shout this from the mountaintops, but they do state it somewhere buried deep in the tech notes section of their support pages. Unfortunately I didn't discover that before buying.
I unknowingly had a VIA chipset on my FW interface and had somehow managed to get the MOTU working for a while (had some minor noise issue), completely oblivious of this issue, but the other day it just ceased to work. After a fuckload of (pointless) troubleshooting, I came across that tech note. Bought a new TI chipset card and, what do you know, everything from its installation to its performance work absolutely flawlessly.
This is apparently not limited to MOTU devices (and so might not be news to you - but it was to me, so figured it might be to others) but, from the stuff I've been reading, they seem to be the worst offenders in the audio interface world. So, just a heads up for any prospective buyers.
They are indeed the worst offenders. I've never seen any other company's name crop up quite so many times when it comes to compatibility issues, which is why I avoided them like the plague when shopping to upgrade my recording rig.