merge empty above the startup disk with startup disk trying to merge empty greyed space above my yosemite startup disk to make my entire 500 gb hard drive just one whole disk, instead of 250 gb grey above my 250 gb yosemite startup in disk utility?
The live expansion only works from a lower numbered partition to expand to a larger one. 

If you get the BSD disk number from terminal: diskutil list or the Info button when you select it is disk utility.



assuming you have data on a disk numbered 2 and the free space is 1, you would need to boot to recovery HD (or another OS and perform these steps).


Restore from the Macintosh HD to the free space that will be a lower disk number than the Macintosh HD. (restore is in the edit menu for the app)
Restart to make sure the new copy works.
Erase the old Macintosh HD - the higher number one.
Expand the new partition to take the whole space.


Since this takes several reboots and a copy, if you have a backup it might be faster to just erase the Mac and restore from the backup. I would make a backup before you start just in case - so you won't save any time doing the live shuffle and live restore. If you don't have a backup - I would do that first unless you're comfortable and have done this many times and can spend the time to start new without a backup if anything fails or errors out.