Remounting a drive as read/write crashes the system and no dmesg.boot

[Scott Schappell]

I have a drive (/dev/ad2s1d) mounted to /backup that I want to be read  
only until the backup scripts run and then it will be read/write.  If  
I set /etc/fstab to:

/dev/ad2s1d             /backup         ufs     ro               
0       0

to mount it read only most of the time then do:

umount /backup
mount -o rw /backup

the system crashes, it just reboots about 10 seconds into writing  
data.  The system is perfectly stable with it mounted read/write.

Also, dmesg.boot has completely disappeared from the system, and  
touch /var/log/dmesg.boot it does not get populated.

# uname -a
FreeBSD arthur.silvertree.org 7.2-RELEASE-p3 FreeBSD 7.2-RELEASE-p3  
#1: Fri Aug 14 13:27:47 PDT 2009     root at arthur.silvertree.org:/usr/ 
obj/usr/src/sys/ARTHUR  i386

Any suggestions on the remounting drive and dmesg.boot?

Thanks!

Scott

P.S. It's good to be off of FreeBSD 4.11 :)