How do I format a partition with the GUID partition scheme?

I have a WD external drive. The disk has three partitions. I need for one of the three partitions to be a bootable disk with Mac Yosemite OS X. Before I do that, I need to change the partition scheme to DRUID.

How do I do that using the Mac OSX Disk Utility?

Many thanks

I don’t think these drives are bootable to run an OS from it if that’s your goal.