Thunderbolt Target Boot Mode

Macs still have this incredibly useful, not very well-known feature: target disk and target boot mode. In combination, they allow you to boot one Mac using the boot partition of another Mac. Why is that useful? Well, here’s the number one use-case: If you have a MacBook – a portable work computer – but need more processing power (or a quieter workspace), you can hook up your MacBook to an faster desktop Mac and work with all the data and preferences just as if you’d be working on your MacBook. This is also useful for working with sensitive data that shouldn’t be copied to another drive as your Data essentially remains on the other boot computer’s drive.

How it works

In order to use target mode to boot another Mac, you need to do two things: using target disk mode on the Mac you want to boot from and then booting your other Mac from the drive attached.

What you need

You’ll need a cable and potentially adapters.

  • If both of your Macs have a Thunderbolt 3 port, use a Thunderbolt 3 cable. Even though they look the same (apart from the lightning bolt icon), Thunderbolt cables will give you a performance boost over using a USB-C cable on supported Macs.
  • If one of the Macs has a non-thunderbolt USB-C port (like the 12” MacBook), it doesn’t make a difference if you use a Thunderbolt or USB-C cable.
  • You can also use Thunderbolt 1 or 2. Look for the lightning icon next to the port as the Mini DisplayPort built into some older Macs looks identical to the Thunderbolt 1/2 port but does not work with target mode.
  • Last, if you have an even older Mac (like a 2010 Mac Pro or MacBook Pro), you can use a Fire FireWire 800 cable (or FireWire 400 for really old macs).
  • You can use adapters form thunderbolt 2 to 3. I haven’t tried Thunderbolt to FireWire adapters, however.

Booting into target-disk mode

The fastest way to boot your Mac into target Disk mode is to shut it down and restart it while holding the “T” key on the keyboard. If the computer you’re booting from is a MacBook, make sure that it’s charging as Target Disk mode uses significant power.

You should then see a Thunderbolt icon on the display.

Booting via target mode

Now, shut down the computer you would like to boot from the other computer an re-start it while holding the option (⌥) key. You should then be able to select the external drive (yellow icon) using the arrow keys and select it with the return key. Sometimes, it takes a few seconds.

Known issues

There are a few problems when using target boot mode:

Mostly, DRM-protected applications need to be re-activated (including Apps from the Mac AppStore).

Also, Time Machine gives an error message when both the internal and external drive are named the same “e.g. “Macintosh HD”. After exiting target mode and rebooting normally, time machine backups continue as normal.

Performance also degrades, depending on the connection used. The slowest is possibly FireWire, but Thunderbolt also only works really well on newer, Thunderbolt 3 equipped Macs. The fastest I got in read/write speeds via Thunderbolt target mode was around 500MB/s when using a 2017 iMac pro to boot from a 2016 MacBook pro. That’s decent, but about a third of the 1.5GB/s you can get from the SSDs in the MacBook Pro and about sone-sixth of the insane 3GB/s the SSD RAID in the iMac pro can push.

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