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You'll get USB 3 speeds with a SSD and slower with a platter, not the worst on older versions of OS X. What are you trying to boot?
 
Most USB3/SATA docks ARE bootable.
But perhaps not ALL of them.
The only way to know, is to buy it, try it, and see.

All of the docks I've bought (last 2 were from plugable.com) have been bootable.

My guess is that you'll do fine.
 
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