Apple Hardware Disk Utility Unallocated Space on my Hard Drive
OS X Yosemite automatically converts hard drives to Core Storage volumes, whose sizes can't be changed. You need to undo the Core Storage conversion.

Here are instructions for making that happen. (source - this method worked for me). This method should be non-destructive, but as always, take a backup first! 


Boot into Recovery HD. If for any reason you don't have Recovery HD, then you'll have to make a bootable Yosemite drive and boot into that.
In the Menu Bar, select Utilities > Terminal.
Run diskutil cs list. You'll get an output that looks like this (credits to the Ask Different user in my source):





Select your Logical Volume ID (the one highlighted in the screenshot) and copy it to the clipboard.
Run the Terminal command diskutil cs revert Paste-Your-Logical-Volume-ID-Here.
Run diskutil cs list to verify that your computer can't find any CS volumes.
You're done! Now Disk Utility should be able to resize your HD!