I recently had a similar problem. I created a data partition and I had to use gksudo nautilus to create folders on that partition. That gave ownership to "me" with group "root" and "others" only having access not to create and delete files. And so when I copied files into those folders as standard user I could only access those files. I could not write to them.
The answer is to use gksudo nautilus to select the folder, select Properties and change the permissions in the permissions tab. You will find that with gksudo nautilus you get an option to change the permissions of files inside each folder. It can be done one file at a time but this option will change the permission of "group" and "others" for all the files. Look for the button that says "Change permissions for enclosed files."