It is generally a good idea to use full-disk encryption, especially on a laptop. However, this becomes bothersome if you are trying to re-partition your disk, say, to make the system dual-boot. Here’s a quick process that I used on my MacBook Pro.

1. Reboot and hold down Command-R to boot into the recovery image.
2. Open the terminal and review your disk partitions:
   

   diskutil cs list

3. Find the one that says “Locked” and unlock it with your root password:
   

   diskutil cs unlockVolume UUID -stdinpassphrase

4. Now you can initiate the decryption process:
   

   diskutil cs revert UUID -stdinpassphrase

5. Check on the status of your partitions:
   

   diskutil cs list

Note: You can perform the same process via the disk utility GUI. Keep in mind that the decryption process will remain in a paused state until you reboot into standard image. Then you can go to System Preferences -> Security & Privacy -> DiskVault to monitor progress. This may take about an hour. Once the process is finished, Command-Q out of System Preferences and go back in to get current status. It should say that disk vault has been turned off.