This is a simple but not a straight forward process. You may need to do this for several reasons, I needed it for successfully booting OsX Mountain Lion and Windows 8 with a shared third partition. If you want to remove the “Recovery HD” partition, you’ll need to do a little bit of work because it’s a hidden partition.
Finally, if you don’t know what you’re doing, don’t play around with partitions, diskutil, merging, or anything else, you could easily screw something up and lose all of your data.
Deleting the OsX Mountain Lion 10.8 Recovery HD partition can be achieved in a very simple way. There are a few ways to go about doing this, all methods will result in data loss. We will be deleting and merging a partition with diskutil from the command line. This is the most precise method I know of to remove the partition since it targets the recovery disk directly and merges it with the full Lion partition – if you’re not comfortable with the command line this is not for you.
diskutil eraseVolume HFS+ Blank /dev/disk0s4
diskutil mergePartitions HFS+ Lion disk0s3 disk0s4