I am not the owner of my external drive

If you are going to use sudo with graphical applications, such as nautilus, than you really ought to use gksudo. I don't have enough rep to comment, but this is important because it is a security risk to run graphical programs with normal sudo, so thought I'd mention that. A better approach would be to open a terminal and navigate to the mountpoint, probably somewhere like /media/$user/device , and than run ls -l to figure out the permissions.

Regarding the original question, what kind of file system is the external drive formatted to? You can find out by running as root fdisk -l I have had this problem before with a VFAT partition. Even when logged in as root I could not write to the partition. Sometimes this can be caused by corruption on the disc or partition, it's very frustrating.

I am still looking for an answer to the VFAT no write access issue myself.

Use the following command in terminal.

sudo nautilus

And enter your password.

This will open file manager with root permissions.

Now you may access your external hdd and make changes to it.

Note : Don't close terminal until you are finished making changes to hdd

One thing to check: Make sure you're not viewing the folder through the admin:// protocol. It looks like this in the breadcrumbs of the Files app (nautilus):

If it shows "Administrator Root", press Ctrl+L and remove the text admin:// at the start of the address, leaving one slash (/media/...), and hit Enter.

It should now show "Computer" as the first item:

The admin protocol can cause permissions errors and warnings when accessing files and folders you normally have access to; I don't know what causes it to rear its head, but that's how you chop it off.