Target Disk Mode

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Target Disk Mode (sometimes referred to as TDM) is a boot mode for Macintosh computers wherein the Mac does not load the operating system, but instead behaves as a FireWire mass storage device with the SBP-2 (Serial Bus Protocol) standard. ^[1]

When booted to TDM, all of the computer's attached volumes (HFS+ volumes, MS-DOS volumes, DVD-ROM, etc.) appearing as devices attached to the hub. It is possible to daisy chain several Macs together by booting them each to Target Disk Mode. All of the volumes on all of the computers will be available to the host computer at the end of the chain.

1 History
2 Using Target Disk Mode
3 Requirements for Target Disk Mode
4 Notes

[edit] History

Apple introduced disk mode access with the PowerBook 100 and continued to offer it with most of the subsequent PowerBook series and Firewire equipped Macs. As long as the requisite software appeared in the system ROM, the Mac could be booted into disk mode.

Originally called SCSI Disk Mode, a special cable allowed the original PowerBook series to attach to a desktop Mac as an external SCSI disk. A unique system control panel on the PowerBook was used to select a non-conflicting SCSI ID number from the host Mac.

With the change to IDE drives starting with the PowerBook 150 and 190, Apple implemented HD Target Mode, which essentially enabled SCSI Disk Mode by translating the external SCSI commands via the ATA driver. Officially reserved for Apple's portables only, all PowerBooks exclusively supported disk mode except the 140, 145, 145B, 150 and 170. However, SCSI Disk Mode can be implemented unofficially on any Macintosh with an external SCSI port, by suspending the startup process with the interrupt switch, as long as its internal drive can be set to a different ID than the active host System's devices.

When Apple dropped the SCSI interface starting with the PowerBook G3, Firewire Target Disk mode replaced the earlier disk mode implementation. Further, TDM now officially supports all desktop models as well as the portable lines, if equiped with Firewire ports. In addition to the previously mentioned 68K PowerBooks the only Macs which do not provide for any kind of disk mode (supported or otherwise) are the original iMac and iBook series, as well as the new MacBook Air.

[edit] Using Target Disk Mode

Target Disk Mode is activated by holding the T key on the keyboard immediately after powering on the computer. The Target Disk booted computer is then plugged into a host computer or other device via a FireWire cable. To shut it down, you simply eject the volume(s) from the host computer and then press the power button again.

[edit] Requirements for Target Disk Mode

[edit] Host computer requirements

The host computer (the computer into which the Target Disk Mode booted computer is plugged) must meet the following requirements:

Built-in FireWire port, or a FireWire port on a PC card
FireWire 2.3.3 or later ^[1]
Mac OS 9 or later ^[1]

It is also possible, though difficult due to idiosyncrasies in the way the TDM Mac emulates a mass storage device, to connect a Mac in Target Disk Mode to a Windows PC. It often works the first time a Mac in TDM is connected and fails on all subsequent attempts. By uninstalling the 1394 device from the Windows device manager, and then reinstalling it, the Windows PC will often recognize the Mac disk again.

[edit] Compatible computers

eMac (all models)
iBook (FireWire) and all models introduced after September 2000
iMac (Slot Loading) with Firmware version 2.4 or later
iMac (Summer 2000) and all models introduced after July 2000
Mac mini (all models)
Mac Pro (all models)
MacBook (all models)
MacBook Pro (all models)
Power Mac G4 (AGP Graphics) with ATA drive
Power Mac G4 (Gigabit Ethernet) and all models introduced after July 2000
Power Mac G4 Cube
Power Mac G5 (all models)
PowerBook G3 (FireWire)
PowerBook G4 (all models)

Note that this list includes all Macintosh computers introduced after July 2000, excluding only the MacBook Air.

[edit] Similar Utilities

On a non-Macintosh computer, the users can often share their internal drive using a Live CD, such as BART PE or Knoppix. This is not as convenient, but provides the means to copy files off an internal drive even if that drive does not contain a bootable operating system.