Not having any similar device, I'm not sure what to suggest. I have a feeling this might hal interfering, since it's used in 5.8 SOHO, but not in 5.8 Standard. Anyway, I can't really think of a reason why a USB device shouldn't be able to charge all the time it is plugged in, specially while mounted. Only if the kernel shuts down power to the USB port can I understand it, but that definitely doesn't happen while mounted. Must be some design limitation of the device itself.