Root password problems

I have a few Debian boxes, and a FreeBSD box, that no one knows the root
password for. I have booted with an init=/bin/sh then tried passwd. This has
worked only on one of the boxes. The rest get an error message of:

passwd: Authentication token manipulation error

My research shows that I should backup the shadow file and run the pwconv
command to regenerate the shadow file. I can not do this on these machines
because they have mounted the root file system as read-only and will not
allow me to update the shadow file. What do I have to do to fix this problem
so I can run the passwd command on these machines?

David