Why does my 2.5 TB HDD appear as a 2.2 TB HDD and a 300 GB HDD? How do I merge them?
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.