When I spoke to Meridian recently, the official line was still 2Tb for MD600 but they were testing other drives.
The problem is not just size & read/write speeds, there are clearly other potential issues that may affect the way the hardware in both TwinStore and MD600 deals with some drives. Heat monitoring being just one.
Meridian are careful to "only" rubber stamp drives that they have proven experience with. Understandable I guess.
No one is suggesting that MD600 won't run with bigger drives, just that, currently this is uncharted water and there have been problems with one or two drives that they've tested (Sorry, I don't know which ones).
So I guess the bottom line is .... if you try something larger it's at your own risk (currently)... but I think you know that anyway.
My own personal experience has not been great with the WD caviar green range. They seem fairly reliable but we have found the performance to be lacking, particularly when using them in other NAS units for Video as well as audio. All 7200rpm drives (Seagate and Hitachi) performed much better.
Gerry