"You can't resize the partition on this disk because it uses the Master Boot Record partition scheme

I am trying to get rid of one of the volume ("mountain Lion", the second from the top) of the partition of an external hard-drive. In disk utility I get what you can see in the pic, namely it does not allow me to modify the partition saying "You can't resize the partition on this disk because it uses the Master Boot Record partition scheme". Do I have to repartition the whole thing or is there a work around? (I am running Yosemite)

Thanks,