start up disk space..Help!

when i purchased my G5, the store partitioned the hard drive with 15 GB on start up drive, remainder on HD2. All applications reside on HD1. Everything has been fine for years, but I am suddenly experiencing all kinds of errors when trying to edit audio tracks in imovie. I only have 1.5 GB available now in HD1, so I am guessing that this is causing the problem? I have tried to get rid of everything I could (even moving things off of HD1), but can't get rid of or move other things "without permission". Then other programs started failing (itunes couldn't find my library), Quicktime won't share in expert settings.....things are beginning to snowball quickly!
I have 3 external drives, 2 firewire, one USB, where I have tried to move most of my imovie projects to. Called the guy at the local mac store and he told me to move evrything i could off of HD2 and edit projects off of firewire drive, because the internal drive "was working too hard". Does this make sense? I did it and it did not improve my audio editing problem. If I try and adjust audio at all, (move along timeline, fade, delete) i get spinning wheel for a few minutes. Forget about trying to zoom or play anything while in timeline mode. Help me please!

Apparently the minimum free space on a boot drive is 20 percent or so, to allow the system to use virtual memory and do the rest of it's housekeeping chores.

If you've cleared that much or more space from the drive, do the fsck -fy thing in that old post. After it's done and you restart the machine, start Disk Utility from the version in the Utilities folder in your computer, and run the permissions repairs.

The drive was working hard because the small space it had left forced it to keep overwriting the files it had to create.