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Mac OS X includes an extremely useful boot utility called Target Disk Mode Target Disk Mode allows a user to transfer data from one Mac, to another Mac, via a FireWire or Thunderbolt connection.
In this tutorial, I'll show you how to set up your hardware for Target Disk Mode and then use it in three different scenarios.
Setting Up The Hardware
Start by setting up your hardware to use Target Disk Mode. Target Disk Mode is only useful for sharing data between two Macs, so this tutorial assumes that you have two Macs that need to share data (for applications which we will discuss below). It is important to understand the terms target Mac and host Mac. Simply put, the target Mac is the Mac you are transferring data from, while the host Mac is the Mac you are transferring data to.
Tip: The target Mac is the Mac you use as an external hard drive and is started up first. The disk of the target Mac is available to the host Mac and files can be copied from target to or from the host.
The first thing you will need to do is determine what connection you will use to link your two Macs. Target Disk Mode fully supports both Thunderbolt and FireWire connections (though not USB).
If you have two recent model Macs with Thunderbolt ports, and you have a Thunderbolt cable, Thunderbolt is the obvious choice for Target Disk Mode because of its blazing fast data transfer rates. (Note, Thunderbolt was introduced on the 2011 MacBook Pro and has come standard on every Mac hardware update since. If you have purchased a Mac in the last two years, there’s a good chance it has a Thunderbolt port, but you may want to check the specs on your Mac just to make sure.
If both of your Macs do not have Thunderbolt ports, the next option for connection is FireWire. Assuming both Macs have a FireWire port, you will simply need a Firewire cable to link the two Macs.
You may find yourself, however, in a scenario where you are linking an older Mac that has FireWire with, for example, a brand new MacBook Air that does not have Firewire. If this is the case, you will need to purchase Apple’s Thunderbolt to FireWire adaptor in order to use Target Disk Mode.
Once you determine which connection you'll be using for Target Disk Mode, the next thing you need to do is power down your Macs and connect them using the appropriate cable. Also, if your target Mac is a MacBook of some sort, Apple recommends that you plug it up using its AC power adaptor before booting into Target Disk Mode.
With the Macs powered down and connected, via Thunderbolt or FireWire, you are ready to boot your target Mac into Target Disk Mode. To do this, press the power button on your target Mac and then immediately press and hold the T key on your keyboard. Within a few seconds, your target Mac will boot up and display a light grey screen with a floating Thunderbolt/FireWire icon.
Now that your target Mac is booted into Target Disk Mode, we are ready to look at three scenarios using Target Disk Mode and how you should boot your host Mac in each case.
Scenario 1. Using Your Target Mac as an External Drive
Target Disk Mode allows direct file transfers from your target Mac to your host Mac, as if your target Mac is simply an external hard drive connected to your host Mac. Using this feature is extremely simple. All you need to do is power on your host Mac as usual and log in. Once your user loads and you go to Finder, you will find your target Mac’s internal drive listed on the side of Finder like it is an external drive.
This use of Target Disk Mode could come in extremely handy for anyone that deals with large files, like video editors for example. Rather than waiting to transfer dozens of gigabytes of video files to an external drive, and then waiting again to transfer those files on to your secondary Mac, you could simply use Target Disk Mode as a direct connection between your two Macs for a one-step file transfer.
When you are done transferring files, you can simply eject your target Mac like you would any other external drive.
Scenario 2. Booting Your Target Mac’s OS on Your Host Mac’s Hardware
If you boot your host Mac into Startup Manager while your target Mac is connected in Target Disk Mode, your target Mac’s drive will show up as a bootable volume on your host Mac.
To boot into Startup Manager, press the power button on your host Mac and then immediately press and hold the Option key (sometimes marked Alt) on your keyboard. Within a few seconds, your host Mac will boot up and display all of the bootable volumes on or connected to your computer, one of which will be your target Mac’s internal drive.
Select that volume as the OS you want to boot, and a few seconds later, you will see the user login screen from your target Mac load on your host Mac. Your host Mac is now successfully running your target Mac’s OS.
There are several applications for this use of Target Disk Mode. For example, an interface component of your MacBook may have suddenly died. Examples of this would be the screen or the trackpad. Using another Mac, you could boot the broken Mac’s OS on a fully working Mac and recover files you were working on, do a system backup before sending off your Mac for repair.
Scenario 3. Using Target Disk Mode with Migration Assistant
Migration Assistant is a standard Mac OS application that helps transfer a user’s data from their old PC/Mac to their new Mac. If you are transferring from an old Mac to a new Mac, Target Disk Mode in combination with Migration Assistant will be the fastest and simplest way to transfer your data and settings.
To use Target Disk Mode with Migration Assistant, all you need to do is power on your host Mac as usual and log in. Once your user loads, open up the Migration Assistant application, which is found in Applications > Utilities.
When the app loads, it will ask you how you want to transfer data. Select From another Mac, PC, Time Machine backup, or other disk and then click Continue.
You will then be asked to enter your admin password, and then given an option on where to transfer data from. Select From a Time Machine backup or other disk.
On the next screen, you will be able to select the internal drive from your target Mac as the drive to transfer from. You will be able to customize what exactly will be transferred in the process.
Once you click Continue, Migration Assistant will begin transferring data from your target Mac to your new host Mac. Once Migration Assistant is finished, you can eject your target Mac.
In this tutorial, I have shown you how to set up Target Disk Mode and then use it in three different scenarios. By following these simple steps to properly boot your target and host Macs for your particular use, you’ll gain access to some incredibly useful Mac OS X features that simplify and speed up data transfer between your two Macs.