FireWire Target Disk Mode: Target Computer Shuts Down at Startup
FireWire target disk mode requires the target computer to have an ATA hard drive at ATA bus 0.
When you try to start up the target computer, it makes its startup sound, but then shuts down. No video appears on the screen, nor does the FireWire icon. The computer starts up normally without the T key held down.
Make sure the target computer has an ATA hard drive on ATA bus 0. It is normal for a target computer to shut down if it does not detect an ATA hard drive on ATA bus 0.
All later Macintosh computers come with an ATA hard drive at ATA Bus 0 in their default configurations. However, it is possible to custom order (CTO) a Macintosh that does not.
58583: "Macintosh: How to Use FireWire Target Disk Mode"