Bob,
Are you using iDefrag for this (you had mentioned it in an earlier thread)? I downloaded the trial which scans only and was debating buying it.
Yes, iDefrag it is. I made it through Tiger and Leopard without the need for defragging. Upgraded the hard drive when I hit 40% free space under Leopard.
Started storing more videos, books and the new higher bitrate Audible books on my Mac instead of the server with SL. Plus, I seriously move a lot of data on and off the system drive on an ongoing basis.
My first upgraded drive, I started noticing a slow down that could not be fixed by any other method when I hit 30% free space. After running iDefrag I was back to a smooth running machine again.
Thus far, I have only run a full defrag a total of 4 times in a year and 1/2 since I started using it. (Probably once every 6 months is all I've needed.)
Once was after a SuperDuper! clone to a new drive. Discovered SD! does not put your system files at the front of the drive. iDefrag did move them all back to the beginning of the drive - reduced boot time by 8 seconds for me among other things.
What I have seen so far, when I start noticing a slow down not taken care of by cleaning the drive to get back to 40% free space and running Onyx, is the free space fragmentation # has been over 50%.
My guess... It might need to be run more often on drives running at 25% or less free space in combination with a habit of moving a lot of files (inc'g video) on and off the drive.
While it's helped me (because I'm extremely picky about my system running smoothly), I'm still of the opinion that the majority of typical users will never need to defrag their drives.