Yet More Info: In Device Manager, I see an "Unknown USB Device (Device Descriptor Request Failed)" for the USB2 port where the Scarlett is plugged in. I unplug the Scarlett and plug in something else (a disk drive), and Device Manager is happy with what got plugged in.
If I plug Scarlett into a different USB port, THAT port goes to "Unknown USB Device" in Device Manager. Plugging something ELSE into that port makes Device Manager happy with the port again.
Rebooting my computer does not change this situation at all.

Have you tried installing the Focusrite driver software with the device in a USB2 port?

Thank you, but that wouldn't make any difference. You install the drivers BEFORE connecting the Scarlett to the computer, not after it's already connected.

I would try to uninstall the drivers, reboot, and install them again. Then try to plug into an USB2 port. It could be worth a try..
All the best.

OK thanks, that was worth a try.
Had no effect on the problem, though. Still get an instant crash in Sonar if I try to play through the Scarlett. And after Sonar crashes, Scarlett disappears from the Device Manager, and unplugging/replugging it gets "the last device you connected to this computer malfunctioned, and Windows does not recognize it."

Any word from Focusrite yet?

Nothing from Focusrite yet. But if they're on the US East Coast (are they?), then my support request was late yesterday afternoon. I'm still watching & waiting.
You have this problem too?

Have you installed the drivers with administration rights? (right click, "Run As Administrator")
 

Hmm.  I know the cardinal rule in troubleshooting is that it is always software/drivers and rarely hardware,  but that does sound suspiciously like a hardware problem. (I had a usb dmx controller that failed and gave me the same error).  Fingers crossed it is not though.   Have you tried the interface on another Pc just to rule that out? 

ernietamminga
Nothing from Focusrite yet. But if they're on the US East Coast (are they?), then my support request was late yesterday afternoon. I'm still watching & waiting.
You have this problem too?

No, I've been running with Focusrite interfaces since 2007 without issue, but it seems like you might well have a rogue unit.