All 2012 MacBooks have USB 3.
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HERE Apple claim speeds of 5 GBPS on USB 3.
I don't understand your contribution. USB 3 is capable of 5 gbps not (GB). There is a difference between gigabits and gigabytes.
Quoting the maximum speed of USB 3 is not really adding anything to this discussion. That's the speed that USB 3 CAN transfer at; it does not magically mean that every USB 3 device can achieve that. A 2.5" 7200 RPM hard drive can transfer data at around 110 MB/s. A USB 3 small capacity flash stick is a lot slower. USB 3 is capable of around 450 MB/s.
A transfer protocol is like a road. Just because a road is safe for 450 km/h, it does not mean that any Trabant can now all of a sudden do 450 km/h. It will still only be able to do its normal maximum speed.
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Which drive are you using? That's the speed it's supposed to be.
What do you mean "supposed to be"? There is no "supposed to be" for a USB 3 device. It depends on the device. If I put in a really fast SSD drive into a USB 3 enclosure, then I can get the top USB 3 speed (around 450 MB/s in sequential reads and writes). If I use a RAID 0 USB 3 enclosure with two 7200 RPM 3.5" drives, then I could get around 220 - 280 MB/s.
If I use a 2.5" 5400 RPM hard drive then it will do around 60 MB/s. And it does not matter if I put this drive into a thunderbolt or USB 3 or Firewire 800 or eSATA enclosure.
Edit: The figures above are averages and for illustration purposes only. Some drives are slower; some drives are faster. The performance of a USB 3 device is dependant on the speed of the device itself and the controller that it has implemented. For example some USB 3 external HDD enclosures are better than others, that's why they don't all cost the same.