6
From Wikipedia:
The (USB3) "SuperSpeed" bus provides a transfer mode at 5.0 Gbit/s additionally to the three existing transfer modes. The raw throughput is 4 Gbit/s, and the specification considers it reasonable to achieve 3.2 Gbit/s (0.4 GB/s or 400 MB/s) or more.
Even that, though, is far faster than what a single drive is likely to be able to manage.
USB3 does also move from half-duplex to full-duplex communication, so that should improve latency significantly over USB2.
My guess is that if you're talking about a single drive on each bus, you wouldn't see a lot of difference between them. But that's just theoretical and based on my limited understanding of USB.
What about SSDs? – soandos – 2012-07-12T04:28:58.400
1@soandos: SSDs can saturate both interfaces. In such a case, SATA 6.0 Gb/s will outperform USB 3.0. – bwDraco – 2012-07-12T04:35:02.013
@DragonLord, this was more of a prompt for wfaulk, but yes, I believe that is the only case where it makes a difference – soandos – 2012-07-12T04:39:40.727
45Gbit/s is the speed of a wire, not the speed of a disk. It seems unlikely that you could saturate the link speed of the USB3, or SATA cable with a single disk. – Zoredache – 2012-07-11T23:55:44.533
So from the users point of view, the external USB3 disk is just as fast as the internal SATA disk? – Sandra – 2012-07-12T00:04:51.920
3My guess is that you should see no difference. USB3 is probably going to have more overhead since it needs to support devices other thing storage. Also consider that aside from flash drives almost no hard disks actually have a USB connection on them. A typical USB hard disk is a SATA disk with a USB-SATA converter in it. That converter could add additional latency and slow things down. So I suspect the answer is that it depends on how well the USB to SATA converter works. – Zoredache – 2012-07-12T00:09:49.700
Unattractive for what purpose? – Andrew – 2012-07-12T02:11:17.770
@Zoredache, not true for high end SSDs – soandos – 2012-07-12T04:28:12.457