FWIW I've just had to return a WD20EARS too.
SMART was reporting the drive was fine but write speeds fluctuated badly, all the way down to 4MB/s at some points (read was still around 100MB/s). I ran the preclear script three on the drive and SMART eventually showed that lots of sectors were being remapped. After the preclear, a smart 'long' test failed so it went back to WD.
If you can, remove the drive from the array and preclear it a few times, it takes a while but it confirmed a drive problem for me after wondering if it was the port/cable/motherboard.