Resizing a SSD partition/volume impossible?!

Hello Folks!

I got a problem with my MBP early 2011. Two Years ago I installed a 1TB Samsung SSD because the original HD cr***d out on me...

I installed the drive and formatted with the Mountain Lion Disk Utility into two partitions

250 GB Macintosh HD

750 GB Data

Afterwards I did a Clean Install of OS X ML onto MacHD and transferred all the Data (Music, Pictures, Movies, personal Files) to the Data Partition.

All worked fine!

When Mavericks came out I split the "Data" Partition into 500 / 250 GB and cloned the existing ML system from Macintosh HD onto the new 250GB Backup-Partition.

Then clean installed Mavericks onto Macintosh HD. After a while and since everything went perfect it was time to delete the Backup System und re-assign the 250GB to the "Data" Partition. So I deleted as one does the Backup Partition with DiskUtil and resized the "Data" Volume to the former 750GB.

Everything worked fine!

When Yosemite came out I did the EXACT same procedure. Splitting, Backup-System, Clean Install on Macintosh HD, testing a few weeks...

UNTIL

It was time to free the Backup Space again...

So I deleted the 250GB Backup Partition as always. But now under Yosemite I can't resize the "Data" Partition to its former 750 GB. I am stuck with 250 GB unallocated Disk Space wich used to be the Backup Volume. I can neither merge it to the Data Partition nor format it to use it as a third Volume! The space is there but I can't use it!?

Has anyone here had the same problem or knows a solution? Other people suggested to reformat the whole drive but that seems a bit excessive to me? I used to just drag the little lower right corner in DiskUtil to the Volumes former 750 GB glory and hit apply. That's all. Very Apple. But now I can't anymore. How is that possible?

Please help I'm going mad... :-(

THX

Niko

This is how it used to look thus it's not a Screenshot from my System.