Can't copy files

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http://askubuntu.com – Help. I'm a complete noob. When I right click on a file or folder the cut/copy options grey out. I tried using $gksu nautilus But when i tried to change the file permissions it just reverted back. Some times it says "Error setting file permissions. Read only file system" or something like that. Do i have to remount the drive as R/W or is there any other way??? A link would also be sufficient.. (HowTos)