* according to installer syslog, /dev/sr0 is already mounted.

 Mar 14 07:34:03 preseed: running preseed command preseed/late_command: in-target /media/cdrom/preseed/one-postinst.sh
 Mar 14 07:34:03 in-target: mount: block device /dev/sr0 is write-protected, mounting read-only
 Mar 14 07:34:03 in-target:
 Mar 14 07:34:03 in-target: mount: /dev/sr0 already mounted or /media/cdrom busy
 Mar 14 07:34:03 in-target:
 Mar 14 07:34:03 in-target: mount: according to mtab, /dev/sr0 is already mounted on /media/cdrom
 Mar 14 07:34:03 in-target: