My Book Studio II (2TB - RAID 1) - Keeps shutting off

I’ve now had my My Book Studio II (2TB as a 1TB RAID 1 device) since February 2010.  It is my Time Machine Backup drive.

It has been working perfectly until about three weeks ago when I noticed that it was switched off.  I pressed the button on the back to switch it on again and thought no more about it.  

Several days later it was off again.  So I again pressed the button at the back to switch it on again.

More recently the off state appears to be gradually increasing in frequency so that now it is shutting off almost every hour or so. I can turn it back on again of course, but this behaviour is negating the purpose for which I purchased the device, namely for my periodic Time Machine back-ups.

This problem has sort of crept up on me as I described above so I can’t imagine that it is a ‘setting’ that needs to be adjusted.  Is this a fault with the drive? Or is this the power saving method referred to in the sales blurb?

I am using the MyBook Studio Edition II (4 TB, Raid 0) on a Mac Pro (OS 10.5.8). I am experiencing the same issue. The drive shuts down after a Time Machine backup, and at seemingly random times otherwise, and needs to manually started.

This was not an issue previously, and the drive would stay in sleep mode with Time Machine backups completed flawlessly.

I have had this drive since July '09. As far as I know, it was working perfectly until five or so weeks ago (?)

The problem predates OS 10.5.8.