Burning an ISO from the terminal?
 

How can I do that? I only have a server installation and need to burn an ISO to a NOT empty cd-rw.

You need to use the cdrecord command e.g.
There are other important options so read the man page.

I have some example commands on This page. I think for a CDRW, you can combine the commands to create a multisession CDRW if you want, and there is also the commands for blanking a CDRW there.

I don't know what is installed in an Ubuntu server install, so you may need to install cdrecord and mkisofs:
"apt-get install mkisofs cdrecord"

I hope this helps
--Ian