It looks like I have made myself a bit of trouble. I recently bought a new 500 Gb hard drive (SSD) and partitioned and formatted half of it to use for Ubuntu 12.04, saving half unallocated for later in case I would be able to add another Linux (e.g. Ubuntu MATE).
Now GParted seems to be telling me that I can't use the unallocated half of sdb unless I "use Partition menu options, such as unmount or swapoff, to deactivate all partitions on this device before creating a new partition table." And I can't do that with an active partition.
Does this mean I have to kill my installation of 12.04 before I can do anything else with the disk? I'm guessing the only way to do this is to go back to the Ubuntu LT install disk and use its GParted to reformat and partition the whole HD, but that idea hurts. I've got a lot of hours invested in /dev/sdb1.