WD NAS hangs after a couple days

I have a wd20000h2nc-00 NAS. I successfully mapped it as na network location. I can read and write to it. I don't use it very often though. The problem is that after a day or two after restart I can no longer access it. Nor as a network location nor via web interface. It looks like it's going into some kind of sleep mode or something. After I restart it again, everything goes back to normal.

I'm not sure if it's due to inactivity time. The firmware is up-to-date.

Any ideas, whats wrong?