Connect a Mac as an external drive to another Mac
Macs can boot into a “Target Disk Mode” that causes them to function like an external hard drive. Connect one Mac to another Mac and you can access its files in the Finder.

Before entering Target Disk Mode, you’ll need a few things:


Two Macs: Target Disk Mode works with Macs, so you’ll need two Macs for this. Each Mac needs either a Thunderbolt port or a Firewire port.
A Firewire or Thunderbolt Cable: You’ll need either a Firewire cable or a Thunderbolt cable for this. You can’t do this via a USB cable. If one Mac has a Thunderbolt port and the other Mac has a Firewire port, you’ll need a Thunderbolt-to-Firewire adapter cable.
FileVault Disabled: Macs now enable FileVault encryption by default, which will prevent you from accessing FileVault-encrypted home directories over Target Disk Mode.
No Firmware Password: If you’ve set a firmware password in the recovery environment, you’ll need to disable that first.


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How to Enter Target Disk Mode


Connect your Macs via a Firewire or Thunderbolt cable.
Click the Apple menu and select System Preferences. 
Click the “Startup Disk” icon and click the Target Disk Mode button to restart your Mac in Target Disk Mode.
You can also enter Target Disk Mode by rebooting your Mac and holding down the T key as it boots.