Device Offline - not accessible message

I am having a similar problem - the disk keeps going offline periodically and then reconnects again after about 5 or 10 minutes, although it often needs to be triggered somehow. It happens when I am on my Mac on my home netwok, and also when I am on my iPad out of home, even when in the middle of downloading something, so it is something to do with the disk not staying online all the time, and not to do with going to sleep when not in use. Anyone got any ideas?