Splitting basic volume (disk) will not work for me?

I need to split a 500GB drive into two volumes, I go through all the steps, it shows me what going to be, then I commit. Then it tells me a reboot is required, I click continue, the machine reboots but nothing changes, the drive is the same as it was before. Am I doing something wrong or missing a step? Is trhere a solution? Need help, please.
Thank you