Advertisement Every Mac can use a Your Mac has several startup key combinations that unlock a variety of boot modes for troubleshooting. Here's a guide to what they all do. One of these is Target Disk Mode, which essentially turns your Mac into an external hard drive. By connecting two Macs together in this way, you can quickly transfer files, migrate your data to a new Mac, or access your startup disk when macOS refuses to boot. While, Target Disk Mode provides added peace of mind in case disaster strikes. Let’s take a deeper look at what Target Disk Mode is and the different ways you can use it to your benefit.
Target: This is the Mac that contains the disk you want to access. You won’t be able to do anything using this Mac, since it will remain in Target Disk Mode for the duration of the operation. Host: This is the Mac which will be accessing the drive. It will boot into macOS as normal so you can transfer files. Connect Your Two Machines. Old Mac OS X, cannot get admin access, boot safe or single-user mode, nor reinstall. Making various changes via 'System Preferences' (asks for admin account). Mac Pro can't safe boot. Unable to boot into anything but single user mode.
What Is Target Disk Mode? Target Disk Mode is a boot mode which allows you to browse and transfer files to and from a Mac’s internal drive without booting macOS. Volumes mount virtually and instantly, and the use of a cable means that transfers are significantly faster than equivalent wireless methods. You cannot use the target Mac while it is in Target Disk Mode. Your Mac essentially becomes an enclosure for your internal drive.
In order to use your Mac again, you’ll need to disconnect and reboot as normal. Target Disk Mode was first introduced with the PowerBook 100 in 1991 and has made it into most Mac models since then. Exceptions include the tray-loading iMac, Power Macintosh G3 and G4, models of iBook G3 without FireWire, the first MacBook Air (2008-2009), and old unibody MacBook. What You Need to Use Target Disk Mode You’ll need two compatible Mac computers in order to use Target Disk Mode, each with a FireWire or Thunderbolt interface.
You’ll also need a cable and any necessary adapters (like Thunderbolt to FireWire, or Thunderbolt 2 to Thunderbolt 3). You cannot use plain old USB type-A connectors (not even USB 3.0), but old Thunderbolt and FireWire connections play nicely with the latest standards. 
Be aware that Thunderbolt cables aren’t cheap. For a 2.6-foot-long Thunderbolt 3 cable. If you’re using a recent Mac, like the post-2017 MacBook Pro or slim iMac, make sure you pick a genuine Thunderbolt 3 cable and not a new-shape USB cable (or Apple’s charger). We’ve put together a guide to help you Wondering what USB-C and Thunderbolt are, and how these types of cables affect your MacBook? Here's everything you need to know about your MacBook ports.
How to Use Target Disk Mode on Mac When using Target Disk Mode, each Mac takes on a different role: • Target: This is the Mac that contains the disk you want to access. You won’t be able to do anything using this Mac, since it will remain in Target Disk Mode for the duration of the operation. • Host: This is the Mac which will be accessing the drive. It will boot into macOS as normal so you can transfer files. Connect Your Two Machines Take your cable and connect both computers via the relevant Thunderbolt or FireWire ports. Connect any adapters you need for older machines. If you’re performing this operation on a MacBook, make sure it has enough power for the duration of the transfer or connect it to a power source.
Start the Target Mac in Target Disk Mode You can do this two ways: • Shut down your target Mac, hit the power button, then press T and hold it while your Mac boots. You can let go when you see a Thunderbolt or FireWire icon on screen. • If your target Mac is already running, head to System Preferences > Startup Disk and click on Target Disk Mode to force a restart into Target Disk Mode. No need to hold any keys down here. Decrypt and Access Your Drive Wait for macOS to detect your target Mac’s drive. If your target Only by manually encrypting the files on your hard drive can you truly keep your files safe. That's where the Mac OS X FileVault comes in., you’ll need to enter your password when you start up the target Mac.
Wait for the drive to decrypt, then it should show up like any other external drive. Copy, Transfer, and Disconnect Use Finder to browse files, copy to and from the drive, and then safely eject your drive. You can do this by dragging your target Mac’s drive icon over the Trash can, or by right-clicking the drive and selecting Eject. On your target Mac, press the power button to power down the machine. You can now restart this machine as normal, if you want. When to Use Target Disk Mode on Mac Now that you know how to use target disk mode, you should familiarize yourself with some of the applications for this boot mode. Quick Wired File Transfers If you’re used to transferring files between computers using intermediary media like an external hard drive, why not use Target Disk Mode instead?