"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,
You should be able to delete the partition, and you would be able to re-use the space to create a new partition (of the same size).

For some reason (not technical) OS X (lion or later) won't let you resize partitions on MBR disks. You could delete using gparted from a Linux live CD or many partitioning tools.

You don't say what you are proposing to use the partition for, or what is on the "Files" partition. You won't be able to boot from a MBR disk, but you can backup an OS X partition. NOTE I haven't tried this on Yosemite which now creates Core Storage partitions.