Thomas Nikolajsen <thomas.nikolajsen@mail.dk> added the comment:
Could you try booting with AHCI driver disabled (menu item at boot),
and test if this makes you able to modify MBR of your
<SAMSUNG HM251JJ 2AA00 00> disk (currently ad0).
If this doesn't succeed; could you try DragonFly/i386
(I see you are using DragonFly/amd64),
to see if this makes any diffference being able to modify MBR.
If still no success, try disabling AHCI using DragonFly/i386.
_____________________________________________________
DragonFly issue tracker <bugs@lists.dragonflybsd.org>
<http://bugs.dragonflybsd.org/issue1560>
_____________________________________________________