Partition with half full external HDD

Hi everyone!

My external Harddrive was half full when i decided to make a partition on it.
I am on OS X 10.11.5

I use Disk Utility and choose my harddisk. Pressed the Partition and on the graphic indicating the partition I made a decission of share it into something like 600Gb and 1.4TB.

Then the process started and got to Resizing where it still is. Allmost an hour later.

How long can this take? Have something gone wrong? Can I loose the data I had on the HDD?
If something is wrong, what will happen if I stopped it somehow? (unplugging, forece quit?)

Best, Michael