First, I would check the command-line
diskutil list to see what the partitions look like. The command-line also shows hidden partitions like your Recovery partition, so there are chances that other non-visible or damaged partitions are shown there as well.
Then, depending on what you see, you can try to either expand the internal partition, merge two (or more) partitions or clone the current partition onto an external hard drive (if you have one available, or at least the disk space), then reformat the internal drive and put the clone back. The latter two options have a high chance of you losing the local Recovery partition. While 2011 and later models of computers do have the ability to directly boot from the Internet, some may want to keep a local version as well. So, use it at your own risk.
To expand the volume, try the following command:
diskutil resizeVolume <disk_slice> 120G
<disk_slice> is the disk you see on the same line of the name of the disk in the list output (e.g. disk0s2).
After you ran this command, you need to visit System Preferences > Startup Disk to make this volume the boot drive again.
To merge partitions, you can use the following command (as stated before, this can result in losing your recovery partition, if it is in the range of to-be-merged partitions):
diskutil mergePartitions JHFS+ Macintosh\ HD <1st_disk_slice> <last_disk_slice>
If you have spaces or other non-alphanumeric characters in the name of your partition, escape them using the backslash ("\") character as shown above for
For any of those commands, you can get help by typing just the first part(s) in:
diskutil (showing the valid "verbs"),
diskutil resizeVolume (showing help on how to resize) and
diskutil mergePartitions (showing help how to merge).
Finally, if you have a Mac OS Extended-formatted external hard drive (not in use or at least with sufficient disk space) available, you can see about cloning the internal drive over, then re-partition the entire drive and clone it back. This will, with almost 100% certainty, remove the Recovery partition. So, again: Use at your own risk.
Here is how:
- start Internet Recovery (Cmd+Alt+R at boot)
- open Disk Utility
- choose the volume (by default named Macintosh HD)
- click on New Image at the top of the window and choose the external hard drive as a destination
- once done, choose the disk above (not the volume), choose Format from the right-side tabs, provide a name (Apple-default is "Macintosh HD"), then format the drive
- choose the volume again (it should now be again 120 GB in size) and choose Restore from the right-side tabs
- click on Image and choose the image from the external hard drive
- select the image on the left side navigation bar (it should be placed in there now) and choose menu "Images" > "Scan Image for Restore" (this can take some time)
- once finished, drag the disk image into the "Source" box to the right and the "Macintosh HD" into the "Destination" box
- Double-check that you have it this way and not the other way around
- click on Restore and confirm it (I recommend reading that confirmation dialog as well, so that you are sure you do the right thing).
- Restart the computer once done