WD doesn’t give that information for some reason, so without opening the case is not possible to know.
That’s why I asked here (in addition to opening a ticket with WD support – which has had no response). I thought that it was possible that some user(s) had opened the case of one or more of these products (after the warranty had expired … or not) and had looked inside.
I suppose one reason for the official silence is that WD may use whatever model drive they happen to have the most of at any particular time – sometimes using a Caviar Green drive, other times using something else.
In addition to any potential reliability differences between different WD drive types, there is also the question of whether a particular drive uses 4K Logical Sectors, which will not work with system images created by Windows 7 backup (although there is a WD fix for that: http://tinyurl.com/bvgwzzx). But maybe only WD drives > 2.5 TB use 4K logical sectors, so that’s not a problem for the 1 and 2 TB products.