Why won't disk utility let me remove a partition from my external hard drive? I have an external 1.5 TB hard drive split evenly with a partition for backup and a partition that was used for Time Machine on my previous computer (OS X Lion).

Now I'm using this hard drive with a new computer (OS X Lion), and I don't want the Time Machine partition anymore. I should also point out that I've never configured Time Machine on my new computer.

I'm trying to remove the Time Machine partition, but the minus (-) option isn't available. Does anyone know why or how I could get the (+/-) options un-grayed?
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