In the transition to Apple Silicon, starting in November 2020 with Macs with the Apple M1 processor, target disk mode was replaced with Mac Sharing Mode. You would simply connect your Firewire or Thunderbolt cable so that the two Macs are connected, and then follow the steps below for setting up Target Disk Mode.
Once connected, you can see the drive as an external hard drive named Macintosh HD. Once you finish, your Mac will restart and start the Target Disk Mode tool automatically. Target disk files allow you to share files between two Macs that are connected via FireWire, Thunderbolt 2, USB-C, or Thunderbolt 3. Release the keys when you see white text on the screen. Once this is done, click on 'Target Disk Mode' and then click on 'Restart'. Start or restart your Mac and immediately press and hold command-S on your keyboard.
Your Mac will restart and show FireWire or Thunderbolt icon to indicate that it is now operating in Target Disk Mode. In this window, by clicking on 'Authenticate' you can set a password or Touch ID to protect your data during the process. Target Disk Mode If you want to recover files from unbootable Mac, then Target Disk Mode boot option is best choice. If you use Option + N keys, then you can start from a NetBoot server using the default boot image provided.
SCSI Disk Mode appeared in the PowerBook 160 and 180 in October 1992 and was supported by most subsequent PowerBook designs until SCSI drives were phased out in favor of IDE drives in the PowerBook 190, which used HD Target Mode to convert IDE signals over the external SCSI port. In System Preferences window, select Startup Disk icon and click Target Disk Mode button to restart your Mac in Target Disk Mode. Hold down the N key while starting Mac to boot from network, in case your setup has compatible network server. However, it was not supported by other early PowerBook models designed by Apple (such as the PowerBook 140, 145, 145B, 150, and 170). Target disk mode was originally introduced in October 1991 as SCSI Disk Mode in the PowerBook 100, designed for Apple Computer by Sony.