WD 8tb Elements NAS keeps unmounting?

Hey all, first off am a total rookie so anyhelp at all is appreciated!

I have a Asus RTN66R router that supports turning a USB device into a NAS.

I have connected my WD 8tb Elements to it which is formatted as NTFS and it all seems to work great for a day.

Next day it seems to unmount itself? Have plugged this into the PC and can confirm it does the same after a day its unmounted.

How do I stop this happening? Is this a sign of a faulty drive? Its only 2 months old.

Thanks again for any help at all!