USB 3.0 vs USB 2.0 for external Hard disks drives

USB2 running in "hi-speed" can theoretically do about 60MBps (MegaBYTES) maximum (AKA 480Mbps), but you'll never get that.

If you are using both drives on the same USB hub (as it appears in the picture), then you are sharing that available bandwidth between the devices. So at theoretical best (with both drives being accessed) you'd probably only get like 30MBps to each drive. After adding overhead and real-world physics to the mix, then getting ~15MBps (BYTES) sustained sounds about right to me.

You're not going be able to reassign which USB ports are attached to which hub, without some very good soldering (at least). :)

You're going to want to get that 2nd drive onto a different USB hub, or hooked up via Firewire, eSATA, or alike to get it onto a different data bus.

I have an USB 3.0 2.5'' Drive (without external power source) from verbatim and when I plug it into my old laptop which only has USB 2.0 it is incredibly slow (much slower than my USB 2.0 2.5'' hard drives of comparable size). My guess is that this has something to do with the power, but I haven't ruled out that it might be some other issue, as I'm not sure how to check that. Needless to say, it runs just fine and much faster than my USB 2.0 drive on a USB 3.0 port.

So, my (statistically not significant) experimental result is: It works, but it might be slow, and I'm not sure if it is good for the hard drive to run on too low power.

And about how to find out: I tried to find out about that by looking at the technical details and comparing them, but there was nothing in there that seemed helpful. If I remember correctly, there were other USB drives that explicitly stated that they work on USB 2.0 ports, and mine didn't, so maybe that is something to look out for, although it also might be a case of asbestos free cereal.