Why does my 2.5 TB HDD appear as a 2.2 TB HDD and a 300 GB HDD? How do I merge them? I have a Western Digital 2.5 TB hard drive. (Model WD25EZRS) It's connected through an external USB dock to my iMac, running Mac OS X Mavericks.

In Disk Utility there are two volumes, totaling 2.5 TB but separate as 2.2 TB and 300 GB. How do I format / partition / group the volumes to use this was one large drive with one partition?

Adding image http://mattmanuel.brace.io/diskutil_for_stackexchange.png
Open Disk Utility (in Application/Utilities). Locate your harddrive on the left. Not one of the partitions, but the whole harddrive. Then click Partition in the tabs on the right (see screenshot). Then click on the the dropdown menu (where Current stands in the screenshot), and select 1 partition (see screenshot 2). Click apply, and wait. Then you have 1 volume.

Note, your data will go lost. Save it somewhere else. Then you can safely put it back, after it has become 1 volume.