In freebsd-10.0 the /etc/rc.d/jail script has been changed to convert jails defined in /etc/rc.conf to the jail(8) format and then start a jail using those jail(8) definitions. The /etc/rc.d/jail script logic is incomplete and inaccurate in what and how the on the fly conversion is done. The show-stopper problem is the fstab statement in the /etc/rc.conf file is not being converted to the jail.conf format. On inspection of the /etc/rc.d/jail script shows there is no logic to convert the fstab statement at all. Only the 3 most basic jail definition statements are converted and a bunch of other un-necessary statements are inserted into the jail(8) format file which is written in /var/run/jailname instead of /etc/jail.conf like the man page states is the correct location. In addition the /var/run/jailname jail(8) format file contains these un-necessary statements. exec.clean; exec.system_user = "root"; exec.jail_user = "root"; allow.set_hostname = 0; allow.sysvipc = 0; In effect this show-stopper forces the 10.0 user to immediately manually convert to jail.conf method or restore the 10.0 system back to a previous release of his system thus negating the intended purpose and results of the auto conversion. I am sure this is NOT the original intended purpose. Fix: Logic has to be added to the /etc/rc.d/jail script to convert all possible rc.conf jail statements as shown in /etc/default/rc.conf to the jail(8) format and remove the un-necessary statements it now inserts. An alterative would be to restore the 9.2 /etc/rc.d/jail script to 10.0 thus allowing both rc.conf and jail.conf jails to function.
After continued research into this problem I have found that this statement is at the cause of the problem. jail_legacy_rc_dir0_flags="-n legacy_rc_dir0 -l -U root" I comment that [flags=] statement out and everything works as expected. My previous diagnosis was incorrect.