I'm using a 74GB WD Raptor HDD and a 500GB Seagate drive. All my music and data is on the 500GB. First I noticed iTunes stopped playing then I noticed the HDD was no longer on the desktop or Finder. I rebooted and as soon as it do that, I can hear several click sounds as it's starting up. When it gets to the desktop again, the clicking continues every ~15 seconds with 3 clicks each time. I've only had this HDD for 1 1/2 - 2 years too.