Pro Tools Takes Forever To Close Sessions

[lenkarm]

I searched on the forum but couldn't find an answer to my efficiency-eating problem

My problem is that once my external hard drive disk space gets into the range of being 33% used, my Pro Tools sessions take forever (~10-15min!!!) to close. When I close a session, the computer is very slow (pinwheel of death) for about 7 min before the "Closing Session" progress bar appears. When I first start using a drive, everything works properly, but as I use it more, this problem arises. I've tried Western Digital and Seagate drives using Firewire 800 and they both display the same irritating problem.

A bit of background, I'm running very large sessions (~96 tracks with many plug-ins instantiated) and using a G5 Power PC Dual 2GHz and running Mac OS 10.5.6. Any suggestions would be extremely appreciated. Thanks so much for your time and interest!