When and How to Use Target Disk Mode

Target disk mode is a Mac feature that has been around since the release of the PowerBook 100 in 1991. It allows users to connect two Macs together via Thunderbolt, USB-C, or FireWire ports and share the content of the ‘targeted’ Mac’s internal drive.

Target disk mode can be used to:

• Copy files from one Mac to another via cable instead of sharing the same local network.
• Verify the integrity of the targeted Mac’s startup disk and troubleshoot any problems.
• Use an optical drive on the target disk mode Mac as if it were attached to your computer.
• Troubleshoot your Mac by booting it from the target disk mode Mac.

To get started, you only need a few basic things:

• The right cable to make the connection between two Macs. target disk mode doesn’t support connections that use Apple USB-C Charge Cable, USB-A to USB-A cable, or the Mini DisplayPort cable. You need a Thunderbolt 3, USB-C, Thunderbolt 2, or FireWire cable to establish a connection.
• Access to power. While it is possible to use target disk mode on laptops, to avoid a sudden drop in connection due to a drained battery it is better to connect your MacBook to a power source while using it as a target disk mode Mac.

How to boot into target disk mode

There are two ways to boot into target disk mode, either via a keyboard shortcut or via System Preferences. The easiest way is to start the Mac and press the T key down until you see a FireWire or Thunderbolt icon on the display. That, however, is only possible if your keyboard is working. On a wireless keyboard, press the T key only after hearing the chime (if applicable).

If your Mac is already running, you can use the system preferences pane to reboot it into target disk mode. This is especially handy if the keyboard is not functioning correctly, as you can rely only on the mouse.

• Launch System Preferences.
• Select the “Startup Disk” pane.
• Click on the lock icon in the bottom left and type in the admin password when prompted.
• Click on the “Target Disk Mode” button to restart the system into the desired mode.
• Press the “Restart” button.

What to do while in target disk mode

Repair internal drive: Used by long-time Mac users as a method of troubleshooting startup disk issues or to quickly move huge files between Macs (an older, legacy method), target disk mode can be used to repair internal drives that are misbehaving. Connect the two Macs and use Disk Utility to repair the startup disk of the target disk mode Mac.

Move files between Macs via Migration Assistant: This built-in software is great for moving settings and user data over from an old Mac to a new one without the use of iCloud or Time Machine. Migration Assistant will consider the startup drive in target disk mode to be an external drive with a valid system on board.

Boot your computer from the target disk: If you noticed any issues with your Mac, then you can use target disk mode to boot from a secondary computer. However, it must be an identical model with an identical macOS on it, otherwise it won’t work.

1. With target disk mde on, start up the second Mac and hold down the Option key until the startup manager appears.
2. Select the drive you want to start the system to start up from – in this case, the target disk.

An interesting move from Apple

Target disk mode is especially useful when rescuing data from failing internal drives. Considering that MacBook Pros now come with SSDs soldered onto the logic board, it’s wise to have a plan B in case the internal drive or logic board fails.

For MacBooks launched in 2016 and 2017, Apple introduced a data recovery port located on the logic board itself that allowed Geniuses to transfer data from a faulty Mac to a functioning one, but that port was removed from the MacBook Pros launched in 2018 meaning that Apple is cutting the branch out from under itself should a logic board failure occur. The company seems to think everyone will be using iCloud Drive (50GB = $1 per month) so everything will be backed up on the cloud, but unfortunately that’s not everyone’s go-to method.

Use a Mac optimization app to catch up with disk health

As with every issue that you may encounter, there are third-party utilities that can help you to spot a problem before it happens. In addition to keeping your Mac clean of junk and cache data, optimization apps such as CleanMyMac keep an eye on your disk’s health and warn you well before it fails, giving you the time to enter target disk mode and securely move your data onto another Mac.