Check tmp/vfs folder in your JBOSS_HOME/server/<config> dir.
We had some VFS issues before.
What about if you use JBossAS6?
Thank you Ales. I will check this out next time I run into the diskspace issue. Right now there is no folder tmp/'vfs in the 'default' config directory. Last night when the app crashed I added 40GB to /opt LVM to buy some time and hopefully I can make it to Saturday without any major drama. I will keep an eye out for vfs folder as you suggested.
The app that I am using is only supported for JBoss 5.1.0. I am using ([The Oracle] 5.1.0.GA (build: SVNTag=JBoss_5_1_0_GA date=200905221053)