|Most data transfer rates for computer related devices are rated in BYTES per second.|
However, data transfer rates for network related devices including modems, USB devices, and firewire devices are rated in BITS per second.
There are 8 bits to a byte, and in the case of network related devices and USB devices, on top of that, 10 to 15% of the data is used for "overhead". The actual effective data transfer rate for those in BYTES per second is often close to one tenth of what it is in BITS per second, not one eighth .
SuperSpeed USB (USB 3.0) has a theoretical max data transfer rate of 4800 Mbit/s (~572 MB/s).
(1,024mb/gb - 4.6875 Gbit/sec)
In the real world....
"The raw throughput is 4 Gbit/s, and the specification considers it reasonable to achieve 3.2 Gbit/s (0.4 Gbyte/s or 400 MByte/s), or more, after protocol overhead."
The biggest bootleneck on recent computers isn't the max data transfer rate of the cpu or the memory, it's the max data transfer rate of the hard drive the operating system is running from.
e.g. If the amount of data being transfered (in one go) can be completely handled by the onboard memory cache on the hard drive, that's a max 300 Mbytes/sec (assuming you're not using a RAID 0 array).
If the amount of data being transfered (in one go) is MORE than can be handled by the onboard memory cache on the hard drive, the max data transfer rate reverts to the max sustained (continuous) data transfer rate of the hard drive after a short time ( a few minutes at best) - currently that's no more than about 100 Mbytes / sec for recent drives.
The max data transfer rate of USB 3.0 exceeds both of those.
You may not get anywhere near the max data transfer rate in reality.
I certainly haven't for USB 1.x and USB 2.0 rated devices, even for fairly recent systems. .
firewire uses up a lot less data for "overhead"
e.g., for USB 2.0 ....
"Transfer speeds in practice"
"As of 2004[update], the actual throughput of USB 2.0 high bandwidth attained with a hard drive tested on a Mac was about 18 MiB/s, 30% of the maximum theoretical bulk data transfer rate of 57 MiB/s (480 Mbit/s). On Windows, the highest speed observed was 33 MiB/s, or 60% of the theoretical max. The drive could reach 58 MiB/s on Firewire, so the drive's speed was not a limiting factor."
 is a reference to http://www.barefeats.com/usb2.html
"If you want fast external storage then get a computer with an eSATA port and get a eSATA hard drive."
That has a theoretical max data rate of 300 Mbytes/sec (assuming you're not using a RAID 0 array), if the amount of data being transfered (in one go) can be completely handled by the onboard memory cache on the hard drive. In the real world, it may be a lot less, but it's likely to be a lot higher than for a USB 3.0 connection. .