Why am I not able to expand my hard drive partition after deleting Windows?
Try this:


Reboot the Mac into Single User Mode by holding ⌘ (Command) + S when pressing the power button.
When the command prompt finally appears, type:
 /sbin/fsck - fy
After this is completed, type reboot or exit.
Wait for Mac to start up normally.
Back in Disk Utility, hit Verify Disk, then try and partition as normal.