Q: [Partitioning HD] Partitioning Not Completed and 300gb disappeared

Make sure you have a good backup of all your needed data and files. Try to boot in to Recovery and use Disk Utility there to delete your missing space.

About OS X Recovery - Apple Support

Make a bootable USB for your OS X version.

Create a bootable installer for OS X - Apple Support

After you have the installer created, boot from it. Go into Disk Utility, then Partition and Erase your disk drive.

You had a fusion drive that now it is separated into 2.

I would fix the fusion drive if I were you, but it is up to you what you want to do.

Shut down mac.

Press Cmd + R and power on, hold it pass the white or black Apple Logo until it boots completely, there you select Disk Utility, select the HD (first one on the list), on your right click First Aid, there you will have the fusion drive fixed and combined into one, name the partition Macintosh HD.

You can close Disk Utility and back to main menu, choose Install OS X, after it installs you can restore via Time Machine.

AJ