When doing some research for my new, self built NAS, I missed the MTBF information at most drives from Western Digital. I wrote an E-Mail to their support department and asked them to clarify that. Here´s what they told me:

We no longer measure the reliability of our drives using Mean Time Between Failure (MTBF). Our current drive reliability is measured using Component Design Life (CDL) and Annualized Failure Rate (AFR). The Component Design Life of the drive is 5 years and the Annualized Failure Rate is less than 0.8%.

The power saving of this drive is a drive feature, it does not relay on the operating system.

I got in touch with them again to clarify the AFR and CDL. I will update this post as soon as I have news.

//Edit: Here´s whar WD states:

The utilization for this drive is a 24/7 application and 1.2M hours.
The AFR means that less then 1% of these drives manufactured fail annually.