What are the chances?

I bought 2x WD2500JD 250GB Sata drives about 2 years ago. Had great luck with them for about 1.5yrs. Lately they both have been having trouble. I first noticed CRC and Disk errors on one of them, so I RMAed that one. About a month after receiving the new drive, the mate started showing the same signs. Now within about 2 months I have had 4 RMAed drives. The replacements have only lasted about a month each. Now one of the replacements is again showing signs of failure. What are the chances of this happening?

I posted about this about a month ago: http://www.ocforums.com/showthread.php?t=540857
I find it strange that this keeps happening so close to the end of the warranty. All of the replacements have been Refurb Drives. I think there is a conspiracy going on. I plan on calling WD to request a new drive as a replacement.

Any ideas what could be going on? My computer runs about 75% of the time during a normal week. The drives are set to shut off after 1hr of inactivity. They are only used as a temporary storage area for video files that I am working on. What am I doing wrong?

Well, one of my recently RMA'd drives just died on me, and I hardly used it. It is also a WDC 250gb drive, but its the WD2500KS.