A FireWire hard drive that is not bus-powered may not appear on the desktop or in the Finder if it is connected to the computer before it is connected to an AC power source.
Symptom
A FireWire drive that is not bus-powered does become available or appear on the desktop ("mount").
Solution
Connect the drive to an AC power source before connecting it to the computer.
If the issue has already occurred, unplug both the FireWire cable and the AC power, then plug in AC power before reconnecting the FireWire drive.
Additional information
In affected hard drives, the controller logic in the drive activates when the drive is plugged into the computer. The controller logic looks for, but does not find, the drive mechanism as soon as the drive is connected to the computer. The drive mechanism only runs from AC power, so until the drive is connected to an AC power source, the drive mechanism will be inactive. The controller continues to fail to find the drive mechanism even after the drive has been connected to an AC power source.