Same thing here. WD was great in sending out a replacement drive, because my first was experiencing the RAID errors, as well as almost daily dismounts from my iMac under Yosemite, even though I never ejected it (the error was always that it had not been ejected properly, even though it was always mounted). The drive seems to be running fine, and the ejection error seems to be more a Yosemite issue than a hardware issue, but I’m not sure about the RAID error except that Disk Utility and the WD Utility both say the drive is fine.
Either way, my WD Duo is where most of my media files are kept, it isn’t being used as a Time Machine. My TM isn’t big enough to also back up the Duo files, so I’m now using Backblaze as an online backup solution, just in case.
