if you have backed up your personal data.. reboot into recovery mode and reformat the drive - not the partition and it will unite the drive and then reinstall the osHi
I'm selling my Imac (mid 2007) where, once upon a time, foolish me, I partitioned the HD. Now, how do I go to have an unpartitioned HD?
I tried using disk utility, and still had no clue about this.
Finally, I brought the iMac to a repair shop. The HD was dying and had to be changed, which is why disk utility didn't work.