Max2Play is designed and fitted for a specific version of Ubuntu, updating would mess with the system. Regarding size restrictions, we have already successfully integrated a 1TB HDD into the system. This should not be a problem for the U3. Please try it without any additional periphery connected to rule out this origin of error.
Regarding the HDD idle time, I have already addressed this issue in another thread: https://www.max2play.com/forums/topic/hard-drive-goes-into-idle-mode-too-early/
You can try out this solution and change the idle time to your liking.