This question already has an answer here:
Disk Utility.app displays my internal HDD "APPLE HDD" to be entirely taken up by "Macintosh HD" at 759.65 GB. Seeing as it’s a 1 TB hard drive, that’s a little disconcerting.
I also can’t reclaim the space using the partition tool, because there’s no partition to remove.
How do I fix this? Do I need to do a fresh install or reformat or something?
EDIT: Found the answer to it here