Hi Guys ! I'm new here. I was reading and following this topic for a while now. I was waiting for someone to test the WD4000YR under heavy load. Nobody has done this so I didn't knew which drive to get. Because Western Digital does not recommend the WD4000YR in a non Raid System because of the TLER feature. However I bought the WD4000YR and it is running since 2 days under heavy load. I had none system crashes. The drive is blazing fast and it is worth every single cent ! I decided to go with the 5 year warantee. Of course the WD4000KD is not bad, but for those who get later a second WD4000YR can make an Raid array which would not work with the WD4000KD, or not as good. I didn't had any Raid experience yet. The drive is not loud. You can hear it working though, but I have to say my case is opened on the side panel. I use the A8N-SLI Deluxe Mainboard from Asus. I only had the problem that the drive didn't boot the operating system on the beginning, but I think the Cable was not enough attached to it. I was even able to transfer my entire Windows from my old IDE drive to my new WD4000YR SATA drive with Norton Ghost 2003.
There is not much of a difference. The WD4000KD got only 3 years of warantee, and the WD4000KD does not have the TLER feature (won't let the drive fall for long time into a recovery mode). And the WD4000KD doesn't support NCQ. That's all I can think of.