So I was derping around on my computer (it has a 250 gigabyte hard drive) and thought,"hmm I wonder what happens if I partition off 75 gigs of this". believe it or not, it partitioned off 75 gigabytes on my hard drive, so what's the problem? well I have no idea how to make it one big single 250 gigabyte partition again. I tried everything, I did the minus on the partition, I did the slide, I repaired with disk utility, same thing with disk utility from the restart CMD+R, and spent hours on the internet looking for a solution. Is there anyone who can help me here?
You can not do this on an active partition. You need to boot into Recovery Mode which will let you delete the unwanted partition and resize the main partition. Select the whole drive then the Partition tab.
PS Make sure you have a Time Machine backup before doing anything.
You could create another partition that's the size of the other half of the disk. Then, because you can't merge your boot disk, you could make a GParted live USB, boot from it, and merge it.
I had the same problem. I had a 500 GB external hard drive that was partitioned 250GB for OS X and 250GB for Windows. What I did was I copied all of the files that were on my external hard drive to my desktop on my computer. I then opened disk utility and erased the entire external hard drive. This deleted the partition and set it back to 500GB OS X. Once I did this I simply copied my files back from my desktop to the external drive and bingo. Mission accomplished. Hope this helps.