Guess what, my L'pad is working now. The key was to change the USB port. I'll say a bit more because it seems likely you'll encounter this problem again, and maybe something about it can go in the FAQ.
I have more USB-connected devices than ports on my 3-4 year old computer. When the Launchpad arrived I disconnected my MIDI interface (Steinberg MIDEX
, plugged in the L'pad, and all went fine. Of course, I wanted to use the MIDI interface... so, I bought a Targus powered USB hub, reconnected the MIDEX 8 to its previous port, and plugged the L'pad into an unpowered port on the Targus, which has a mix of powered and unpowered ports. Since the L'pad lit up, it did not seem like the powered vs. unpowered port mattered.I've seen USB devices react to relocation of the port in different ways. Sometimes they see the reconnection and the drivers work with no further ado; sometimes the drivers have to be reloaded.
Now that the Launchpad is working, I am going to connect the MIDEX 8 to another port and be done with it. However, it is possible that the issue is not the changed port, but power. Perhaps the L'pad gets enough juice from the USB hub to do MOST of what it is supposed to do, but not all. I could test the theory by plugging it into one of the powered ports on the Targus, but would rather not!
If a FAQ about this gets posted on the Novation support site, it should say something definitive about the power issue, based on testing by Novation.
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So, although I have no definitive answer as to why my Launchpad would not work while connected to an unpowered hub, it is clear that USB port issues can render the Launchpad nonfunctional as a native mode controller in Live.