I've got a FW400 daisy chained to a FW800 drive that connects directly to my MBP FW800 port. This worked just fine for about 2 months and now suddenly the FW800 drive no longer shows up in finder or disk utility. I've reset the PRAM and turned off all drives for 5 mins, shut down all the drives, etc. I'm not sure what else I can do to get this drive to show back up. The only other thing that I can think of to be the problem is the controller in the ext. case - but I've tried plugging into both FW800 ports on the external drive. The kicker is that the FW400 works like a charm. Unfortunately, Apple doesn't have eSata option available and just the one FW800 port.
This happened to my WD 1TB hard drive, but its USB. The drive wouldn't mount at all, nor could it be seen it seen in Disk Utility.
To fix it, I downloaded Disk Warrior (paid application) and it was able to recognise it and 'fix it' to a point that I could retrieve all my data then erase it. That did the trick. I've had no problems with it since (although it is quite loud - not sure if that's directly related to the incident. Ah well, it works.)
I'm not sure there are free versions of this kind of software, but I'm sure a simple Google search will sort you out.
Good Luck!
