Windows diskmgmt won't let me resize storage drive partition

[MatazaNZ]

So I'm trying to shrink my storage drive's main partition by 10 or 20GB to make space for a source control partition to help me better manage my software development. This is how my drives show up in diskmgmt 

And this is what I'm entering when I want to shrink the volume of the E: drive

And when I hit "Shrink", it give me the error "The parameter is invalid". Anyone know how I can fix this issue? I considered something lik GParted, but I don't know if it would be safe, as windows might be trying to protect data, but there is 1.1GB of free space in the drive, and it's 0% fragmented according the Control Panel. 

Any fix is welcome, thank you all in advance

[Speedbird]

Try GParted Live, but back up your Data first.