Target disk mode

Target Disk Mode (sometimes referred to as TDM or Target Mode) is a boot mode unique to Macintosh computers.

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 SCSI, FireWire, Thunderbolt or USB-C external mass storage device.

A Mac booted in Target Mode can be attached to the port of any other computer - Mac or PC - where it will appear as an external device. Hard drives within the target Mac, for example, can be formatted, partitioned, etc., exactly like any other external drive. Some computers will also make their internal CD/DVD drives and other internal and external peripheral hardware available to the host computer.

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. Migration Assistant supports Ethernet (wired) or Wi-Fi, which TDM does not. Neither supports USB; however, Thunderbolt->Firewire, Thunderbolt->Gigabit Ethernet and USB 3.0 -> Gigabit Ethernet adapters are an option when one of the computers does not have Firewire or Thunderbolt.

Apple introduced disk mode access with the original PowerBook 100 and continued to offer it with most 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.

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