Removing an unneeded partition on an external USB drive

I have an external drive that is formatted Mac OS Extended and partitioned into two sections, one that I use and one that I no longer use (and that I had hoped to use as a TimeMachine volume, though that never worked out). I would like to remove the partition and just use it as a single volume, but whenever I try to remove the unnecessary partition, I get an error message "could not unmount disk -69888". Nothing I have done to address this, including using Terminal commands to force unmount the disk, has worked. Any ideas? I don't want to lose the data on the partition that I use if at all possible.