freebsd-update from 8.4 to 9.3 causes gmirror to disappear -- HELP!

[Warren Block]

On Thu, 25 Jun 2015, wfdudley wrote:

> I've been running 8.4 STABLE for a while, but decided to try to use
> freebsd-update to move to 9.3
> I have a pair of 2TB drives in a gmirror.  Works fine.
>
> I used svn to pull the 8.4 RELEASE source, and built myself an 8.4 RELEASE
> system.
>
> Then, I followed the instructions here:
> https://www.freebsd.org/releases/9.3R/installation.html
>
> And after the first reboot in to the 9.3 kernel, my gmirror is GONE.
> This is ALL MY DATA, and the freebsd-update database
> (/var/db/freebsd-update).
>
> After discovering that the mirror was GONE, I did this:
>
> sysctl kern.geom.debugflags=17
> gmirror label -vb round-robin gm0 /dev/ad14

That turns off the safety and creates (writes) a new mirror on one 
drive.  Please stop writing to those drives and check your backups.  If 
you are lucky, the data is still present on the second drive.

> Which is how I configured the mirror initially (from this
> http://www.iprobot.net/linux-tutorial/how-to-setup-sofware-raid-in-freebsd/
> post).

The Handbook has a full section on creating mirrors, but that is not 
what is needed.  Instead, the existing mirror just needs to be mounted.

The problem might be this:
https://www.freebsd.org/releases/9.0R/relnotes-detailed.html#AEN1277

You do not mention what you mean by the mirror being "gone".  Was there 
an error message?  It might be as simple as editing /etc/fstab.