I want to compare the files present in two filesystems or directory trees, and to list which files are present in one tree but not in the other. I don't think 'diff -r' can be used to directly compare the directories because one of the trees is the root fs on which the other fs (a different partition containing a cpio copy of the root fs) must be mounted.

The only way I can think of is to do: 'cd /; find . > file1' with the second filesystem unmounted, then to mount the second filesystem and do: 'cd 2nd-fs; find . > file2', then to compare file1 to file2.
But the big problem here is that 'diff' lists over 700 lines which are different, although most of them are not missing files! Most of them are just files listed in a different order.
If the first files in both filesystems were different, then every line of the 'find' output from one fs will be different from the same line number in the other 'find' output file, and 'diff' will list all lines as different, even though nearly all the files will be the same. So I want to list only the files listed in one file that are not listed ANYWHERE in the second file.

I'm sure it can be done with 'sed', but just the mention of 'sed' gives me a headache - surely there's an easier way? - please!

hello,
i use gtkdiff - which is a very nice version of diff. - nice color coding etc... else i would do as you are i.e. ls > file

How about running the output of find through sort.
Ex: find . | sort > file1