Macintosh HD partition "enlarged" after partitioning failed

Hello,
Yesterday I wanted to partition my 1 TB hard disk (with 300 empty GB) in 2 to create a dual boot : a "large" 900 GB partition with data and the new Mavericks update, and a smaller 100 GB partition with Snow Leopard reinstalled (I kept only Snow Leopard on my computer until yesterday).
So I opened Disk Utility and typed the new partition size, clicked and started the partition... And I realized that Disk Utility didn't keep the same partition size as I wanted and gave 300 Go to the new partition : all the hard disk empty space ! During partition, the process froze, and one hour after, nothing changed. I didn't have the choice anymore, I had to force quit Disk Utility and I rebooted, hoping that it was still too soon to have damaged my hard disk. But I wasn't lucky, when I opened Disk Utility again, it found only one partition (Macintosh HD) but which is 1 TB large, it said the entire partition is busy, there is no empty space at all for a new partition ! I managed to free 100 GB, and Disk Utility recognizes the new empty space, but there are still 200 imaginary "busy" GB on Macintosh HD that I would really like to get back. Even after having rebooted, those imaginary 200 busy GB are still there.
Please, help me to find a solution so I won't have to reset my computer (I bought it in 2008, so it can't be fixed for free anymore) 😟
Thanks for your help !

I apologize if my vocabulary can sound a little bit strange, French is my main language and my computer is entirely in French too, but I know that I can find more answers on forums in English.