Why my Mac is showing only 250 GB out of 500 GB? I have MacBook Pro (13-inch, Mid 2012) 

It comes with a 500 GB hard but today when I checked it it's only showing 250 GB. Please tell me why is that?



Macintosh HD:

  Available: 81.57 GB (81,570,791,424 bytes)
  Capacity: 249.98 GB (249,984,860,160 bytes)
  Mount Point: /
  File System: Journaled HFS+
  Writable: Yes
  Ignore Ownership: No
  BSD Name: disk0s2
  Volume UUID: 805DC50A-074F-3D46-AEE2-0A3974FC8045
  Physical Drive:
      Media Name: APPLE HDD HTS545050A7E362 Media
      Medium Type: Rotational
      Protocol: SATA
      Internal: Yes
      Partition Map Type: GPT (GUID Partition Table)
      S.M.A.R.T. Status: Verified


diskutil list
Check what Disk Utility shows. Probably for some reason the remaining space is not partitioned.

Also, get a good backup since an erase and reinstall should fix things. 

Before you erase, boot to recovery HD and try to repair the Macintosh HD and also repair the drive itself.