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USB 3.0 - why are data transfer speeds not as stated?

Question

I recently transferred a large amount of files from a USB3.0 capable portable HDD and a superspeed USB3.0 cable. When plugged into the USB3.0 slot on my notebook, the transfer rate was shown as only 48MB/s instead of the theoretical 5GB/s. I tried plugging into the 2.0 slot on my notebook, and the transfer rate was shown as 36MB/s. Is there a problem?

Answer

The speed of a USB3.0 external hard drive is limited by following factors:

 

1) the HDD's actual speed,

2) the USB 3.0 to SATA bridge controller's speed,

3) USB 3.0 Host Controller's speed,

4) USB 3.0 Cord's speed,

5) Your file transfer mode's speed.

 


The main reason why the speed is not as high as theoretical best is because of item 2, the USB 3.0 to SATA bridge controller's speed. Since your HDD's native speed is usually no faster than 70MB/s, the bridge controller would take a 20% penalty to translate between the two interfaces.