There are numerous posts already on this issue with no resolution from WD. Check out these posts…
For some reason the EX4100 units periodically stop responding and the only way to get any life back in them is to do a power cycle by pulling the cord out and plugging it back in. I myself have to do the unit I have every 2-3 weeks. Got no idea why this happens and WD are not being forthcoming with a fix.