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Thunderbolt drive support for troubleshooting

Apple's Thunderbolt technology is fast, but people have wondered about its capabilities to both boot and provide Target Disk options in OS X.

Apple's Thunderbolt technology supports both Displayport and PCI Express connectivity through two simultaneous and separate channels, which means it will connect both to displays and peripheral devices such as hard disks, all through the same port at the same time. Given that multiple devices are all connected through one port, people may wonder about the troubleshooting options and limits when making use of this technology for troubleshooting purposes.

Target Disk mode
Apple recently released a knowledge base article discussing the options for Thunderbolt connectivity, which in part covers the options for Target Disk mode. Target Disk mode allows you to boot your Mac in a way so its internal drives can be connected to another Mac as an external disk, which is beneficial for transferring data and troubleshooting boot problems.

The Thunderbolt connection will allow for Target Disk connectivity, but it will also compete with Firewire connections. When in target disk mode, however, even though both Firewire and Thunderbolt can be used, the Thunderbolt connections will take precedence over Firewire connections. Therefore, if you boot into Target Disk mode and then connect the Mac to another one and have a Thunderbolt connection established between them, then that will be used until it is disconnected. There is no way to select which connection to use, so to specify a firewire connection you will have to unplug Thunderbolt devices from your system.

External booting
One concern that people have had over Thunderbolt is whether you can use it to boot to external volumes. Apple has supported doing this for both Firewire and USB connections, but has not been clear on whether or not this is possible with Thunderbolt. Given the support for Target Disk mode over Thunderbolt, it would appear this is possible, and Anand Shimpi from AnandTech recently wrote an initial-impressions article on the Pegasus RAID thunderbolt device, and in response to a reader's question mentioned that the device does boot to the external disk.

This makes sense given Apple's track record for supporting external boot options on Macs, but also falls in line with the features of the EFI firmware that Macs use. EFI is file-system-aware, and since Macs are able to use Thunderbolt in Target Disk mode it means that file systems are available to the firmware through Thunderbolt and do not require software drivers to work. Therefore, the EFI should be able to recognize a boot volume and load from it, regardless of whether it is available via Firewire, USB, or the new Thunderbolt connection.

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