Is the drive still under warranty? If so send it back to get a new drive put in it (just make sure you copy all the data off first!) or get WD to send you a new drive.
Given that its an external drive made by WD, if you're going to replace the drive yourself, I'd suggest making sure that you put the same drive back in, and make sure the drive is running the same firmware as well (avoid any issues) although it may work fine if its just an other WD drive (but don't quote me on it!).
You said it is a 1TB RAID1 drive? If so you'll need a 1TB drive not 500GB. If it was 1TB RAID0 then a 500GB would be needed.