Why don't I have permission to copy to my ext4-formatted external drive?

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http://askubuntu.com – I have an external drive that is a few years old and now I wanna format it. I dont know what file system I should use with ubuntu. I tried ext4, but it gives me a headache when I am trying to copy documents. It says I dont have the permission to copy files there. I dont really wanna use this hard drive with windows computers...that's why I decided to go for the ext4 file system. Any ideas why I cant copy or what could have gone wrong? (HowTos)