I want to split a partition on my external hard drive into two parts. Right now there are two partitions: one for Time Machine and one for storage. I want to end up with three partitions. In disk utility, I select the hard drive on the left side, choose the partition tab, and then select the partition labeled "Storage." Based on what I've read, I should be able to click on the + button underneath that window, but it is greyed out for me. Am I missing a step?