> /sbin/mount -u -w /

> mount: Cannot open '/etc/fstab': No such file or directory
> mount_ffs: root_device on /: No such file or directory

> Hmm, I can't change my file system to 'read write' because 'fstab' is
> missing from my 'etc' directory.

Not quite.  You can't change it to RW *that way* because /etc/fstab is
missing.

You see, mount needs to know both the thing being mounted and the place
it's being mounted on.  The command you gave specifies only one of
those, so mount tries to go off and figure out the other on its own.
First, it tries looking in fstab, but you don't have one.  Then it
tries looking at the mount table in the kernel, and it finds a mount on
/, but the thing mounted there according to the kernel is
"root_device", which doesn't exist (hence the second error line).  (The
kernel sticks that string in there because it doesn't know what /dev
entry corresponds to the root device, though arguably it'd be better to
make a guess that'd usually be right rather than not even try because
the guess might occasionally be wrong.)

You probably want something like "/sbin/mount -u -w /dev/sd0a /" - that
is, explicitly specify the device as well as the mount point.

But you have worse problems.  If whatever you used to install didn't
set up fstab, it probably didn't set up various other things too.
You'll need to find and fix whichever of them are important to you.

I don't know what they are or I'd give you a list.  My own list of
things to do to a new install is:
	- set up /etc/rc.conf.local
	- set up /etc/fstab, make any needed mount points (eg, /kern, /proc)
	- set up /etc/inetd.conf
	- set up /etc/ntp.conf
	- set up /etc/localtime
	- set up /etc/resolv.conf
	- fix root crontab
	- passwd root, add any others (incl. add mouse to groups!)
but this is rather heavily tuned for my own setup.

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