This 'fix' (Resetting the SMC) also works for the iMac, be it in a slightly different way, because there's no battery to be removed. For it to work on the iMac, simply pull the plug lead before holding the power button for >5 seconds.
I have to do this every time I connect any new FireWire device, even though I am still on Snow Leopard. I see no reason to believe the problem lies with Lion, as Snow Leopard also showed these issues. At least in my case. And since I also used a Western Digital hard drive at the time, I know what I'm talking about. My drive was a WD My Passport Studio 500GB. It would stop doing anyrhing at all after transfirring >500MB of files to it, no matter whether I would be in the middle of a transfer or not. Most of the time, the drive would NOT unmount itself, though. It would simply cause a system modal HANG, showing the beach ball and even stopping the seconds counter on the clock. All would then be fine after I'd UNPLUG the WD drive, apart from the obvious warning about the drive not being unmounted properly.
I used an iOmega Ultramax without any problems whatsoever until its power supply kicked the bucket, and I now use an iOmega eGo 500GB as a replacement for the WD drive.
Since the iOmega UltraMax passed away, I've bought a G-Drive (G-Technology). It seems to work quite all right, except that I've received my first random umount from it just a few minutes ago. I'll have to keep an eye on the situation, as I still have the opportunity to return it free of charge, should need be. However, I strongly blame myself for this specific unmount, as I'd been moving stuff around on my desktop and one of the firewire cables is a bit wobbly in the drive (although all Daisy-chained devices still worked, so I'm baffled for it -- the G-drive is currently the first drive in the chain, being connected to my Mac)...
I certainly hope this unmount is the first as well as the last one I've seen from this device. I just wish Apple wouldn't have 'upgraded' the FireWire drivers/stack since Leopard, because all was just fine in those days. It seems, for me at least, Snow Leopard introduced this issue and Lion did not resolve it. Looks as if Apple really isn't giving much for FireWire support any more...