Target Disk Mode is a neat feature of Apple laptops and of recent desktops. It basically enables a computer to act as an external hard drive.

In the beginning...
In the pre firewire days this technology was known as SCSI Disk mode (and later HD Disk Mode). Apple laptops had a special compact SCSI port, for which there were 2 kinds of adaptors allowing you to use normal devices. One kind was for using hard drives, scanners etc. The other kind was for using SCSI disk mode. If you connected your laptop to a desktop using this cable, your laptop would appear on the desktop like a normal hard drive. A control panel allowed you to set the SCSI ID that the powerbook used.

This was a fast and relatively convenient way of transferring files between 2 computers, especially as high speed ethernet cards were far less common in those days. Accessing the powerbook's drive was almost as fast as when using the powerbook itself. It would have been even better if it were not for the constraints of SCSI : no hotplug.

And then there was Firewire
Later Apple ditched SCSI and this feature was reborn as Firewire target disk mode. And Mac users around the world rejoiced and made merry. Firewire makes it even easier. All that you have to do is turn off the target computer, connect it to the host computer with a standard 6 pin firewire cable and start up the target computer holding the 't' key down. No more fiddling with SCSI IDs, shutting down the host computer or hard to find cables !(The SCSI adapters are very hard to find these days). You still have to restart the target computer but booting to target disk mode is much faster than doing a full boot.

These days all Macs have 100 Mbps or gigabit ethernet, but target disk mode remains a viable alternative to ethernet and file sharing as 100 Mbps ethernet maxes out at around 4-5 Mb/s. Firewire on the other hand has a theoretical bandwidth of 400 Mbps and in real world tests I get around 10-15 Mb/s when copying files from my powerbook to a desktop (the laptop's hard disk is probably the limitating factor as it's only a 4500 RPM drive). With firewire it's almost as fast as if the hard drive was in your own computer. It's also very easy to use.
The host computer must be running Mac OS 8.6 or later and have firewire ports (builtin or via a PCI card). The target computer may be any of the following:

  • PowerBook (FireWire)
  • PowerBook G4
  • PowerBook G4 (Gigabit Ethernet)
  • PowerBook G4 (DVI)
  • iBook (FireWire)
  • iBook SE (FireWire)
  • iBook (Dual USB)
  • iBook (Late 2001)
  • iBook (14.1 LCD)
  • iBook (16 VRAM)
  • iBook (14.1 LCD 16 VRAM)
  • Power Mac G4 (AGP Graphics) with ATA drive
  • Power Mac G4 Cube
  • Power Mac G4 (Gigabit Ethernet)
  • Power Mac G4 (Digital Audio)
  • Power Mac G4 (QuickSilver)
  • Power Mac G4 (QuickSilver 2002)
  • iMac (Slot Loading) with Firmware version 2.4 or later
  • iMac (Summer 2000)
  • iMac (Early 2001)
  • iMac (Summer 2001)
  • iMac (Flat Panel)
  • iMac (17-inch Flat Panel)
  • eMac
(list pulled from http://docs.info.apple.com/article.html?artnum=58583
According to the documentation, in the case of the Power Macs, only disks attached to the builtin ATA controller may be accessed (although I can access any firewire drives connected to my eMac). As you can see recent desktops are included in the list. All you need is a firewire cable for hours and hours of endless fun!

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