After spending two weeks fighting with this, I've been able to determine that the root problem is my Toshiba (Portege 3500). I've tried four different Firewire Harddrives and Two different PC Card Firewire adapters, and am continually running into instabilities.
The symptom is that I will be copying a number of files either from the Toshiba to a Firewire Drive or from one Firewire Drive to another linked to the Toshiba. After a bit I get a "Delayed Write Failed" error and then the entire 1394 card and attached device or devices disappear.
I have updated the BIOS to the most recent version on the Toshiba site, and have loaded all the Windows Updates I can find. I have tried setting up attached drives as "Optomized" for Performance or for Quick Removal with no discernable change. I have tried using the Advanced Windows setup to try and give Programs or Background Tasks precedence with no discernable difference based on settings. I've tried setting the Swap file to a fixed, large file rather than System Managed, but that hasn't seemed to help either.
My "guess" is that either there's a WinXP setting I've missed somewhere, or the Toshiba's CardBus implementation isn't up to snuff, or some combination of the two.
ANY help would be greatly appreciated.