USB 2 vs 3

What did I wrote? "many USB 2 soundcards will not work on USB 3", OK? I didn't say "all". I just gave two examples. BTW, MOTU is aware of the problem, and "they are working on it". Focusrite also.
As for the OS, it's Win 7/64. Tested on two different laptops with built-in USB 3 ports.
I know RME did their home work. Fine. But I think you should care about it and not take compatibility for granted, that's all. USB 3 is just "physically" retro-compatible with USB 2. And even then... only up to a point: power. Think about it. Twice, before you burn your MOBO like I did.
I'll just repeat: take care.

Answering to myself: it reminds me furiously about Firewire compatibility some years ago... You bought a PC with a built-in Firewire interface to connect a sound card. And then you're been told that the Firewire chipset had to be this or this brand. And it DIDN'T work with another. And, better still: for this sound card (Yamaha MLan, e.g.) you needed a "TI" chipset", while you couldn't use it for a Presonsus. And vice-versa !!!
I'll never repeat it enough: never take compatibility for granted.

I'm not a professional on the PC side but I'm an audio professional and beta-tester for the likes of Soundscape /SSL since day one (1994). I have some experience about computer music. I had a contact with MOTU via e-mail and they admitted themselves that USB3 was not compatible - yet, of course - with their USB audio interfaces. Same for Focusrite. In the meantime, I still have USB2 ports on the same laptops so, no problem.
Nothing to do with any OS or Mac vs PC or whatever. And regarding the power (or lack thereof) it's in the specs of USB. I documented myself before writing this post. And I think it is worth to just say: "be careful".

If you "google" a little bit, you'll to find this:

http://en.wikipedia.org/wiki/Universal_Serial_Bus#Power

Power

The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines. For USB 3.0, the voltage supplied by low-powered hub ports is 4.45–5.25 V.[47]

A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. A low-power device draws at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws at most the maximum number of unit loads permitted by the standard. Every device functions initially as low-power but the device may request high-power and will get it if the power is available on the providing bus.[48]

Some devices, such as high-speed external disk drives, require more than 500 mA of current[49] and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port. Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power+data, the other for power only) to be plugged into a computer. With such a cable, a device can draw power from two USB ports simultaneously.[50]

I believe Focusrite has a firmware update to fix this issue. Check their website!