Elgato Thunderbolt™ 2 Dock supports booting from USB drives, in certain circumstances.

 

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Computer Needed for Booting

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For booting to work, the connected Mac must be "Late 2013" or later, with one of Intel's fourth-generation Core processors (also known as Haswell Macs).

 

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OS Needed for Booting

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For booting to work, the OS on the boot drive must be OS X 10.9.2 Mavericks, or later.

 

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USB Version Needed for Booting

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For booting to work, the drive can use either a USB 3.0 or 2.0 interface.

 

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Partitioning Needed for Booting

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 For booting to work, GUID Partition Table (GPT) should be used for the drive.