Why am I not able to expand my hard drive partition after deleting Windows? I'm a MacBook user and I had previously installed Windows on my laptop. 

After deleting the Windows partition now, I'm facing difficulty in expanding my hard drive partition to the maximum. After I drag the the partition to Full and click Apply, it shows:


  Checking journaled HFS+ volume


Then, a message pops out:


  Partition failed with the error : couldn't modify partition map because file system verification failed. 


What should I do?
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.