Thanks.
I know RE drives from WD are raid, but they're a bit pricy.
So I've got to stear clear of AS drives for raid if I find seagate cheaper. That's easy enough to understand
In case you don't know - the real difference between raid and non raid drives isn't really the mtbf or yearly failrate, it's how the firmware handles relocations, seek issues and other errors. If the firrmware tries to correct it for too long, most raid controllers will consider it broken and drop it. Then when it recovers half a second later or something, a rebuild will be iniciated. That's why it's important to run raid drives in raid, and not just any desktop drive.
ps. the WD5000YS and WD5000AAKS drives have an expectedly lower AFR - but the YS drive (RE2) is too expensive.