drive won't unmount

[Khutch2]

I have two internal and one brand new external hard drive (firewire). My start up drive (40 gig) is partitioned, one partition is the start up (named Applications) and the other is just for data storage (aptly named Data Storage). The other internal drive is 250 gig. My goal is to "unpartition" my start up drive to restore it as a single drive. The reason was that my partitioned start up drive is maxed out, while the other partition has ~10 gig available.

So, I made a sandbox on my new external firewire drive, backed up each partition of the start up as a disk image on the other internal drive. I booted up on the sandbox and went to disk utility to "unpartition" the internal drive. I got the courage to hit the partition button and got an error message saying the drive Applications will not unmount. Tried several times and always get the same message. Data Storage seems to cooperate. Any suggestions?