Caviar Blue 1TB performance drop

I just bought a Caviar Blue WD10EALX 1TB and the results with HD Tune worry me. There seems to be a big performance drop  at the external side of the platters, and the access times are sometimes anormally high.


I tested the drive yesterday and it was fine (normal continuous transfer rate). Then I filled it up with my data (~600 GB) and now the bench looks like this.

Is it a defective drive and should I send it back ?

yes there is a defective platter or head on the drive

please return the drive