I'm upgrading some systems from ACS 4.1 to 4.2. The first three went fine. On the third the install aborts. I get an error message "Setup failed to export your old user database" and then "Setup could not backup your old configuration. Please check there is sufficient disk space and rerun setup."
1) over 20 GB of disk space free
2) I can do a database export from the 4.1 GUI ok.