Ah. The WD Green drives default to a rather aggressive power-saving strategy that conflicts badly with Linux access patterns. You _could_ be seeing jerky behaviour as a result of Linux having to wait for the drive(s) to wake up again.

There exists a MS-DOS (seriously!) utility from WD that can modify the "WDIDLE3" timeout to a more suitable value (default is 8 seconds, much better is something like 30 seconds).

If you're not into DOS, and not as fussy about warranty, then my own hdparm utility for Linux has the -J flag to do a very similar thing. Eg. "hdparm -J30 /dev/sdX". The settings change need only be done once, and is permanent unless modified again.