I have a Mac Book Pro (late 2011) with 750GB hard drive. A couple years ago I partitioned the disk so I could install a test version of a new Mac OS. (I think it was El Capitan at the time.) The test worked. I updated the main partition to the new OS. A couple years later, I am now running Sierra V 10.12.2. I wanted to get rid of the test partition and merge the space back into my boot partition. So I erased all the data in test partition. However, I am unable to remove the test partition, which was about 180GB, and merge it back into one partition. I just tried it again today using Disk Utility under 10.12.2. This time I got an error message: the partition task returned error code 8 and said it encountered some kind of corruption. Scary. Now the disk utility doesn't even show the test partition. It just the one running partition of about 560GB. So it appears that disk utility won't even let me manipulate the test partition anymore. How can I erase that test partition now and recover the missing 180GB?