I don't know if external drives are treated differently depending on connection type.
It would seem to be the case. Turning on my USB DVD drive it mounts on the fly.
Further testing of the Firewire HDD at reboot as opposed to later on the fly they mounted but Avast 5 dropped 2 elements of its shield:the File System Shield and Web Shield.
I was wondering if I could now turn on the Firewire on the fly to see if Avast 5 had 'learned' it. Well they have mounted alright but the Avast System tray icon has disappeared into another universe. The Program Manager shows the AvastSvc and AvastUI processes still running but I am not sure of a third is missing or not.
Without the system tray icon showing I don't know how to access the features or restart Avast, but opening Avast from the Program menu brought up the UI without duplicating the running processes so that was ok. Trying the 1394/FireWire again on the fly this time Avast still showing in the System Tray and the drive has loaded ... so far so good.. maybe v5 is 'learning' to coexist with my 1394 drive after all! But all of the above instability doesn't inspire overconfidence exactly. Maybe it 'guessed' I was planning to uninstall it due to the other problem I have with its updating.
External FireWire Drives would appear to be handled by v5 quite differently from USB... unfortunately.
(One thing I notice is that Windows FireWall doesn't require flagging USB to allow but it does have such a setting for 1394. So USB and 1394/FireWire is treated differently by Windows XP as well).