FWIW - I just did this using the SDFix app on my stock rooted S5 and it works. The KEY here is to REBOOT YOUR DEVICE after using the SDFix app!
After I ran the app and checked Titanium Backup again to see if I could write to SD, it still said the folder was not writeable. So I downloaded Root Explorer and tried to do it the manual way, but found the xml file was properly changed - which was curious. So why wasn't it working?
That's when I decided to reboot.
After rebooting, all works now as expected! Hope this helps someone!