I ran this on my iMac G4/1.2Gh, iBook G4/1.2gh, PM G5 Dual 2.5 and it worked great everytime. I was amazed that the 1st boot after defrag never brought up the "Welcome to Macintosh" screen, it went from the Gray Apple logo/spinner to my desktop! Talk about new found speed! Apps launch within 1-2 bounces and it protected even my biggest FCE files! I will still use Techtool and diskwarrior to check the HD before optimize, but this program is MUCH better a defrag than TTP4.