Can the default permissions on /dev/disk* be changed?
As far as I can tell, Apple locks this stuff down pretty hard to the point where it's not reasonably possible. 

However, considering what you want to do with it, I'm certain that what you're actually wanting is to give write access to the external HDD which can be found in /Volumes/ (after you plug it in). Now, most external HDDs I've used (that are formatted properly*) will allow anyone to write to 'em unless an Admin chmod 'em to say otherwise. 

*NTFS-formatted drives are read-only through the Mac OS. As far as I know, there is no way around this without reformatting the drive.