I needed to partition my newly bought 1TB WD disk for my HTPC, because i wanted to use some for OS and the rest for data storage, but i had already put 200GB of data onto it. I will explain how I handled this problem, and still kept my data.
First of all i needed a program capable of partitioning a disk that contains data. I was a little careful picking the software, because a software crash right ind the middle of the partitioning, would destroy the data that was already on the disk.
I read a lot of recommendations in different forums and stumbled upon this: http://www.partitionwizard.com/, it did exactly what i needed!
And it was really easy to use, just a few clicks and the disk was partitioned in a few minutes. But remember that it is recommended to backup the data before using the program, even though the program is capable of saving the data.
One thing i struggled with a lot afterwards, was installing the OS and it seemed like i needed to set my newly partitioned part to active(has to be if it should be bootable), stupid mistake but took a lot of time before I realized it was the problem.