My Book Mirror Edition

I have a My Book 1TB Mirror Edition. Every day I need to unplug the power and plug it back in because it unmounts from OS X  and iTunes cannot see it. I do not see any firmware updates available and I did try a new USB cable. Is this a known issue and is there anything I can do to fix this problem? Or is this an RMA issue?

Does the drive enter in sleep mode, or it just unmount without giving you an error message?