How do change permissions on my external HD once and for all?

I’m using Ubuntu.
It’s been happening for some time now that (every once in a while I wouldn’t be able to Write new files or edit/rename others already within the HD.
The first times all I did was restart the system and I seemed to get rid of the problem.

This is the message I get when I try to Cut-Paste an item into my external HD:

Sorry, could not change the permissions of “7343-AD3C”: Error setting permissions: Read-only file system

What must I do to rid myself of this pain in the you know what once and for all?