Hey BSP Fans,
I found this thread really helpful, but I'd like to amend it a little, to reflect my needs and outcome.
You don't need to buy a powered hub. Any cheap, self-powered hub will work.
I wanted to use my iPad with iMPC Pro, to use the DRUM sequence as a sampler. But I got the same message after using the y-cable. I live in New Zealand, and, personally, I don't have the $50 NZD I'd need to buy an entry level powered USB hub. (I just spent $480 NZD on a BSP!!) I needed to find a cheap solution.
Through my research, I found a video on YouTube that said you didn't need a powered HUB at all to bypass the iPad's error message. Just a self-powered hub. Coupled with the y-cable, everything would still get power, and Arturia's reputation would stay in check.
So, I found this locally:
http://www.pbtech.co.nz/index.php?z=p&p=HUBUNI0032&name=UNITEK-Y-2028-USB-2.0-4-Port-Hub-Bus-powered-480MbSomething similar in the US could fetch around $5 USD.
And it WORKED! I was able to bypass the error message. I use SEQ 1 for a Moog Minitaur, SEQ 2 for Moog Animoog, and DRUM on iMPC Pro. I had some issues with one MIDI channel interrupting another, but zero latency. And some cool glitchiness with the BSP's looper.
This technique would work with any hardware that has an external power supply, although I'm not sure how many devices you can use at the same time.
Hope this helps future iPad problem solvers!