For reference only
An incident on 2012-11-23: FireWire GUID ffffffffffffffff is invalid! with LaCie hard disk drives on FireWire, without ZFS, following sleep of hard disks and the computer. Symptoms are fairly predictable.
Sometimes a necessary forced restart of the computer (MacBookPro5,2 with integral FireWire 800) is followed by failure of the operating system to find the FireWire port. In these cases a graceful shut down of the computer allows the next use of the OS to find the port.
The 500 GB LaCie Big Disk Extreme (300794EK) and/or its power supply is known to be problematic so I don't seek help. This topic is solely to demonstrate that problems with spun down hard disks can occur without ZFS.
Incidentally the message from kernel – FireWire GUID ffffffffffffffff is invalid! – may also be seen when the FireWire port is peripheral (not integral) with no device in that port.