Originally Posted by pdxer

the usb port on your computer can only source 500ma which is not enough to charge the ipad and run it at the same time, so when you wake it, you will get that message. if you leave it sleeping, it will charge, but very slowly.

the standard ipad charger is 2 amps, but even a 1 amp usb adapter will be enough to charge & run it at the same time and avoid that message. note that not all usb adapters are compliant with the usb power spec and may not work.