This could be because your user has no permission over the files in your external hard drive.
What I would like to know is if this is the only problem (if you open them as root, the file is opened correctly,
as expected), or if the problem is somewhere else (if you open them as root, even after the chmod has been
executed, they still won't open).
So now try to copy some file with the cp command as root to verify this.
If it works we will manage to get your user able to read and write on that USB device
Could you tell us how your external drive is partitioned too? (NTFS, FAT or ext partitions and such)