I wrote a python script to copy ISO images from my Linux storage server to external USB HDD whenever the HDD is plugged in. Here it is, in case someone finds it useful. Installation instructions are in the beginning of the script.http://shell.jkry.org/~keitsi/misc/wodemate.py
Here's how it works when you setup the script.
Plug in an USB HDD to a Linux computer and the script copies all new ISO images from specified directories to the HDD's ISO directory, with meaningful names. When copying is done, HDD is unmounted.
When you replug the USB HDD, only new images are copied.