What takes so much space in my external disk?
The default permission on .Trashes do not allow for reading.  Use sudo du -ch .Trashes as this will give you both the total and in "Human-readable" output vs. used sectors which you'd then have to convert.

Also you should not delete .Trashes, just empty your Trash.

If you want to take a graphical look at the contents of the disk use Disk Inventory X, you might find it interesting and can do some cleanup from within the App too.