Cannot change sharing permissions of my external disc.

Hello folks,

I've tried many ways but still don't getting any clue how to change it. I know that there was the little lock button in the bottom right corner in information section of any disc. I still have it there in case my internal disc in my Mac, but I haven't it there in case of my external one.

My Mac is in Czech language, but the last sentence about sharing is: "You can only read."

I would be very very thankful for any advice you get me.