Target Disk Mode
![Apple G3 iMac booted in Target Mode](/Images/godic/202502/15/Apple_iMac_in_Target_Mode5921.jpg")
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, or Thunderbolt external mass storage device.