How to Use Target Disk Mode to Boot From Another Mac's Hard Drive


Ken writes: I need to set up and configure a new Mac mini but I don’t have the space at my desk to hook up a monitor, mouse, and keyboard. I work daily with my MacBook Pro; is it possible boot the Mac mini with the MacBook Pro and use the laptop’s display, keyboard, and trackpad to control the mini?


The answer to your question arrives courtesy of a feature called Target Disk Mode (TDM), which Apple introduced way back in 1991 with the PowerBook 100. TDM allows a user to connect two Macs using either FireWire or Thunderbolt, allowing one Mac to access the other’s internal drives as if they were external FireWire or Thunderbolt drives.

TDM is most often used to help diagnose malfunctioning Macs. If a Mac won’t boot, a user can connect it to a working Mac, initiate Target Disk Mode, and then, if the hard drive is still functioning, view the drive’s contents. From a user perspective, the internal drives of a TDM-connected Mac appear indistinguishable from those of an external drive enclosure.

But an even more interesting aspect of TDM is that it allows a user to view another Mac’s hard drive as a potential boot drive, and to boot natively into OS X with a combination of one computer’s hardware and another computer’s software. In Ken’s situation, he would put the Mac mini into Target Disk Mode, attach it to his MacBook Pro using FireWire or Thunderbolt, and then boot the MacBook using the mini’s hard drive, producing the same basic result as if he had physically removed the mini’s drive and installed it in his MacBook.

To accomplish this, first make sure that your “target” Mac (the one you’ll configure for Target Disk Mode) is running a version of OS X equal to or greater than the “host” Mac. As you may know, with very few exceptions, Apple does not update older versions of OS X to support newer hardware. So if your host Mac is a new 2013 Haswell-based MacBook Air, for example, and your target Mac’s hard drive has OS X 10.7 Lion installed, you won’t be able to boot to the target hard drive because Lion doesn’t possess the drivers for the new Haswell hardware.

Assuming that your Macs are compatible, next decide if you’ll use Thunderbolt or FireWire. If available, Thunderbolt will provide the fastest experience, especially when used with Macs that have internal solid state drives instead of mechanical hard drives. With Thunderbolt a relatively new addition to the Mac, however, you’ll likely find that FireWire is far more ubiquitous. Note that TDM will still work if you have to use a FireWire adapter, such as when connecting a 2010 MacBook Pro’s native FireWire 800 port to a 2013 MacBook Air using a Thunderbolt-to-FireWire adapter.

Mac Target Disk ModeIn our example, we'll use a 2012 Retina MacBook Pro (left) as the host to boot a 2011 MacBook Air (right) as the target.

Once your Macs are connected, shut them both down. Next, power on the “target” Mac (in Ken’s case that would be the Mac mini) and immediately press and hold the “T” key on the keyboard until you see a gray screen with a floating FireWire or Thunderbolt logo (depending on the connection type). For Ken’s Mac mini, which lacks a built-in keyboard, this means that he’ll have to at least temporarily connect a USB or Bluetooth keyboard to initiate TDM.

Target Disk Mode MacAfter booting the target Mac while holding the "T" key, the MacBook Air's hard drive appears as a bootable option on the Host MacBook Pro.

Now head to your “host” Mac, power it on, and press and hold the Alt/Option key on the keyboard until you see the OS X boot manager. This screen allows users with multiple internal or external hard drives to manually boot to a specific drive. Since the target Mac is in TDM and connected to the host Mac, the target Mac’s internal drive will appear in this list. Use the arrow keys or mouse cursor to select the target Mac’s drive and press Return to begin the boot process.

Target Disk Mode MacThe Host MacBok Pro is now booted, using the host's hardware to run the target MacBook Air's boot drive.

Your host Mac (in Ken's example, his MacBook Pro) will now boot up as if it was the target Mac (again, for Ken, the Mac mini). Although displayed and controlled via the MacBook’s hardware, the software and operating system will all be from the mini, which is now basically the world’s most expensive external drive. Ken is now free to install software, configure settings, and perform updates to the Mac mini, without having to set it up as a separate workstation on his small desk.

When you’re ready to disconnect the Macs, first shut down the host Mac. Then press and hold the power button on the target Mac to turn it off and remove the cable connecting the two computers.