Lagging on MAC OS X


I am using two WD HDD Studio on my iMAC. And after bit more then half year one of two HDD’s started to lagg badly, whenever i want to enter a folder I need to wait few seconds or even more. The video playback is terible, whenever I want to watch a movie it goes fine then the video stops and after 5-15 sec. it starts again, copying files from HDD is terrible, what is strange both HDD’s were bought about same time, and only one is working not so well, did all the updates and stuff, but still its lagging. Did tests on Mac Book Pro, Windows 7 and the results are same - laggs.