I’ve tried re-installing the firmware (1.2.6) on my DroboFS, but whatever has chewed up the free space isn’t being cleared out by this process.
I need to get ahold of an ssh-enabled firmware so that I can log into my drobo and manually clean the system, and verify that there are no other issues.
Unfortunately, my support contract ran out in May. 
Maybe there’s a friendly person out there that can PM me a url?
I’m pretty sure that a reflash will free all the space on the root partition, since it restores the filesystem at the block level. What won’t be freed during a reflash is the /var partition, but that shouldn’t prevent you from booting.
Are you sure that this isn’t a diskpack problem? Have you tried to remove your diskpack, and then boot with a new disk, just to make sure it isn’t something wrong with your diskpack?
I know that a few people around here were given the “special” firmware, hopefully one of them will help you.