Remove partition and restore the space without erasing
Disk utility can't resize the last partition on a drive.
One way would be to use GParted to move the partition and then resize it.


Boot the Mac with a Ubuntu Live CD.
Run GParted and move the second partition to the beginning of the free space;
Reboot to Mac OS X.
Verify and repair the external disk with Disk Utility. It found a minor error on the volume header of the second partition.
Resize the second partition to the end of free space.


In your case it's a bit easier, since you're only occupying half of the drive.


Create a new partition in the empty space
Clone the second partition to the first one
Delete the 2nd partition
Resize the 1st partition