Reboot the Mac into Single User Mode by holding Command+S during boot
At the command prompt, type:

/sbin/fsck -fy
When fsck is complete, type “exit” or “reboot” to reboot
Boot the Mac as usual, verify the disk again in Disk Utility, and partition as usual

1. Reboot the Mac into Single User Mode by holding Command+S during boot

2. At the command prompt, type:

    /sbin/fsck -fy
   

3. When fsck is complete, type “exit” or “reboot” to reboot

4. Boot the Mac as usual, verify the disk again in Disk Utility, and partition as usual