Why won't disk utility let me remove a partition from my external hard drive?
Disk Utility isn't willing/able to resize partitions on MBR disks. 

It a way, it's telling you now (poorly) that you can only rename or erase the partitions. It's happy to create this scheme, just won't touch it keeping any data intact. 

You will need another tool to make changes to that particular flavor of partition scheme unless you are willing to let Disk Utility repartition and erase all the data. 

I don't see a recent article (or have a drive to test on hand), but it sure was a limitation on Leopard.

Mac OS X 10.5: About resizing disk partitions