I want to backup all system files such as /etc/fstab or resolv.conf that
have been added or altered. I thought I would use
find /mnt/hdb8/ -newer /mnt/hdb8/var/lib/dpkg/info/adduser.list
to get a list of files that needed to be backed up. The above find
gives me an error just like the ls below.

m25@w:~$ locate adduser.list
/mnt/hdb8/var/lib/dpkg/info/adduser.list
m25@w:~$ sudo ls /mnt/hda8/var/lib/dpkg/info/adduser.list
ls: /mnt/hda8/var/lib/dpkg/info/adduser.list: No such file or directory

Can anyone please tell me what I am doing wrong and more
importantly what I need to do to find and backup the files I have
added or altered?

It sounds like your slocate database is out of date. Try running:
updatedb

then your locate command again.

Thanks david_ross,
Maybe we have to do something different when we have
booted from another partition or liveCd?

Your suggestion to run updatedb is something which I
have been known to forget. But just seconds before I already
did updatedb with hdb8 mounted. Locate finds it but
ls and find do not. I can't understand why nor how to
fix it so that they do.

Any suggestions please. And please give me the exact
command to find what is newer than May 10 the date on
adduser.list.

As for the date command - im lost - might check man find
as for the locate/updatedb/ls problem, you'll need to know how it works
ls looks @ actual files and tell you about them, if they're present
locate looks in a pre-existing database too see if a string matching $1 is present, regardless of what your filesystem actually has in it
if you run updatedb (as root) then it will update that database to reflect more current data