making an ISO of a partition

[Lenny]

Hi,
Currently I am using a live version of Ubuntu 10.04 for this task, simply 
because it is the easiest way right now.
I have done this task before, a long time back, but I'm not sure which 
utility did this for me, but I recall that in Ubuntu version 10 or prior, I 
was able to browse to a partition on another disk and I had the option of 
making an ISO image of that partition.
There is probably an easy CLI command for this, and that would be okay too.
I just reinstalled a difficult OS on another drive, with only one partition, 
and I want to make an ISO of the partition as a backup.
So later, if I need to, I can create a new partition on a drive and just 
expand the ISO back onto it.
Thanks for any assistance.

Glenn