Target Disk Mode
When a Mac that supports Target Disk Mode is started with the 'T' key held down, its operating system does not boot. Instead, the Mac's firmware enables its drives to behave as a FireWire or Thunderbolt mass storage device.
A Mac booted in Target Mode can be attached to the FireWire port of any other computer - Mac or PC - where it will appear as an external FireWire device. Hard drives within the target Mac, for example, can be formatted, partitioned, etc., exactly like any other external FireWire drive. Some computers will also make their internal CD/DVD drives and other peripheral hardware available to the host computer via FireWire.
Target Disk Mode is useful for accessing the contents of a Mac which cannot be booted from its own operating system. Target Disk Mode is the preferred form of old-computer to new-computer interconnect used by Apple's Migration Assistant.
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 (SCSI System 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. This also made it possible to select the disk in the Startup control panel and boot up from it.
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.
Thunderbolt supports Target Disk Mode. TDM now officially supports all Mac computer models, if equipped with FireWire or Thunderbolt ports. In addition to the previously-mentioned 68K PowerBooks, the only Macs that do not provide for any kind of disk mode (supported or otherwise) are the original iMac and iBook series, MacBook Air models prior to 2011, the late-2008 aluminium MacBook, and the late-2009 MacBook, all of which lack a FireWire or Thunderbolt port.
Requirements for Target Disk Mode 
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 (original white and black plastic models, not the aluminum or plastic unibody models)
- MacBook Air (mid-2011 and mid-2012)
- MacBook Pro (all models)
- XServe (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 MacBooks with no FireWire or Thunderbolt port, such as the 2008-2010 MacBook Airs and some MacBooks (13-inch, Aluminum/plastic unibody).
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 Thunderbolt or FireWire port (either 400 or 800), or a FireWire port on a PC card
- FireWire 2.3.3 or later
- Mac OS 8.6 or later
- An ATA hard drive at ATA bus 0.
The host computer may run Microsoft Windows, but with some possible shortcomings: to read a Mac's HFS-formatted disks, extra drivers are necessary. Aside from having MacDrive installed, users also must ensure their computer possesses 1394 ports in order to utilize Target Disk Mode methods. MacDrive also has a read-only option to prevent any accidental editing of the computer in Target Disk Mode; however, this mode cannot be set after an HFS/HFS+ disk is mounted.
With the addition of HFS drivers into Apple's Bootcamp, it has also become possible for Macs running Windows to read (but not write) HFS partitions, without the purchase of software. Users have separated these drivers from the main Bootcamp install, and now also install on other Windows computers.
Host computers running Linux are also able to read and write to a Mac's HFS or HFS+ formatted disks through Target Disk Mode. It is working out-of-the-box on most distributions as HFS+ support is part of the Linux kernel. However these filesystems cannot be checked for errors, so for shrinking or moving partitions it is preferred to use Mac OS.
- Apple Inc (16 January 2002). "Target Disk Mode". Apple Developer Connection. Retrieved 13 July 2007.
- Griffiths, Rob. "Borrow an optical drive from another Mac", "Macworld", March 27, 2007. (retrieved October 8, 2010)
- "Macbook Pro EFI Firmware Update 2.2". Apple. Retrieved 15 September 2011.
- Fleishman, Glenn. "Secrets of Thunderbolt and Lion". TidBITS. Retrieved 27 February 2011.
- "FireWire Target Disk Mode: Target Computer Shuts Down at Startup". June 12, 2002. Retrieved 2012-05-22.