This baby saved me much heartache and hair when it accurately predicted a HD failure on my Powermac. I was able to backup everything critical in time. My local Mac repairer confirmed the HD was failing, it was replaced, the backup reinstalled everything and I was VERY happy. That green light is very reassuring and if it turns red - watch out. Best of all it sent me an email to my Powerbook at work, so I knew to check for the failure when i got home.