You'll need to console onto the failed unit and reboot it. We need to get it into single user mode, so when the GRUB screen comes up arrest it by pressing a key, then hit 'e' to edit the GRUB loader. Press 1, to select line 1, then 'e' again to edit it, and add 1 to the end of the line. Then press Enter and hit b to boot the edited loader. This should go through a basic boot cycle, dropping you into a bash shell with just a hash.
From here you can verify if it is indeed a full disk that's causing the trouble, just type df and see if any of the mounts are 100% used.
So far so good, now we need to remount the filesystems so we can clear some space. After much digging in the /etc directory, the following lines should build a usable filesystem for you:
mount -n -t proc none /proc
mount -a -t nonfs
mount -o defaults,noatime,nodiratime /dev/ide/host0/bus1/target0/lun0/part1 /mnt/flash &> /dev/null
mount -o defaults,noatime,nodiratime /dev/ide/host0/bus0/target0/lun0/part2 /original_root/
mount -o defaults,noatime,nodiratime /dev/ide/host0/bus0/target0/lun0/part5 /original_root/var/controller/images
mount -o defaults,noatime,nodiratime /dev/ide/host0/bus0/target0/lun0/part7 /original_root/var/controller/log
mount -o defaults,noatime,nodiratime /dev/ide/host0/bus0/target0/lun0/part6 /original_root/var/controller/log/cdr
From here it's just a simple matter of browsing to the /var/log directory and issuing:
rm -f messages
Type reboot, and you should be good to go.