I have a WD external drive attached to my ASUS RT-N66U. It shows up fine on my LAN and works no problem. Then somewhat randomly, say every 2 - 3 days, the drive will not be recognized by the router, and so I cannot access it from any PC on the network. I have to turn on/off the external drive for the router to 'rediscover' it once more. After that it's fine again, until next time. Internet access is not affected. Any reason why this may happen?