[CentOS] Knoppix won't let me write to emergency drive

I appear to have lost the whole of my home directory on my centos
server (which includes all my mail saved from the last 7 years).
Although I have recent backups of most data I would like to copy
important files from /etc, and, if possible, /var/logs so that I can
try to find out what went wrong.  However, knoppix refuses to let me
write to either my usb hdd or usb pendrive.  Does anyone know how to
persuade it?

Thanks

Anne