Stuck on 'Preparing to partition disk'
This, and other similar problems are commonly fixed with these 2 methods:



Method 1

Go to single-user mode CMD+S on boot). Type the following:

/sbin/fsck -fy


Hit enter, and then type

reboot


and then press enter. Now try making your partition again.

Method 2

If Method 1 did not work, you can boot into Recovery mode (CMD+R on boot), selecting disk utility, then HD and repairing disk and disk permissions. This time make your partition in Recovery rather than in normal boot