According to the WD forums, the EARS WD drives are not RAID compatible. This from the WD forum Community Manager:
Our only Green drives that are RAID compatible are Models: WD2002FYPS, WD1000FYPS, WD7500AYPS, and WD5000ABPS. None of the Caviar Green drives, including the EARS drives are RAID compatible. That’s why they will continue to fall out of the RAID arrays.
Well, I don’t think anybody categorically established that. Data Robotics answers on that subject were always ambiguous. Understanding what TLER does, I don’t see how Drobo could be immune to too long retries, and the lack of TLER is a good candidate for those drives “bad for Drobo” but checking OK with the manufacturer utility.
But I agree with your overall conclusion, the added cost of “Entreprise” drives is not worth it.
None are incompatible (as long as they have the right interface), though there are some that aren’t necessarily recommended, in particular…
AV drives - AV drives sacrifice data integrity for speed, since the assumption is that an AV glitch will show up only in a short period of time, and AV drives are often tasked with simultaneous multi-stream read/write
RE - Drobo doesn’t need (and can’t reach) this level of performance - not yet at least
[*]Black - the lower Drobos can’t reach this level of performance, and the added heat can do bad things like cause Drobo to continuously run its fan at high speed