[TERRIBLE MESS] Cant delete partition, wasted 25 gb space!

The problem is you installed Yosemite on it and now are trying to modify the disk on Mavericks. Basically Yosemite updated the disk drive to support "Core Storage", there's a one liner out there that will let you revert that from the command line. Here: http://awesometoast.com/yosemite-core-storage-and-partition-woes/

(It would be easier to just use Yosemite by the way...)

I may be wrong, as I'm no expert, but a possible answer is to get an external USB hard drive a bit bigger than the Macintosh partition and use Carbon Copy Cloner (CCC) to clone the partition onto the external drive. Then format the entire Mac drive as 1 partition and copy the clone back onto the drive. CCC has a fully functional 30-day trial.

However, take what I've said with a large pinch of salt as I don't have a lot of experience in these matters. Perhaps someone else could chime in and say whether that would work or not.

CCC also allows you to clone the invisible Recovery HD which is important as you will need to put that back onto the formatted drive as well.

I would also suggest backing up your important documents to another location if you attempt the above.

Good luck.

So, even though there is no partition and disk space appears as free, you cannot extend your existing Mavericks partition to recover that 25Go disk space?