subdisk6: detached --> Fatal trap 9

I've been trying to get FreeBSD running on a new machine with the  
following hardware:

	Tyan S2895 motherboard, BIOS v1.02
	2x Opteron 275
	8 GB ECC RAM
	400 GB SATA HDD
	IDE DVD/CD drive

Initially, installation went fine using snapshot 012 from  
ftp.freebsd.org.  However, once the system is up and running, within  
minutes of loading the filesystem with a cvsup or cvs checkout, I get  
errors something like:

	subdisk6: detached
	g_vfs_done():ad6s1f[WRITE(offset=10217842688, length=2048)]error = 6
	ad6: detached
	Fatal trap 9 [...]
	g_vfs_done():[12-15 more messages like the one above.]

Sometimes I get a bit of information about the CPU state, but it's  
usually corrupted.  The machine has failed to drop to ddb most of the  
time, and haven't been lucky enough to get a prompt the last several  
hangs.

Now, the real fun is that after this happens a few times, the machine  
will no longer finish booting, either from the HDD or from the  
install CD.  Booting hangs after probing acd0.  I'm going to have to  
install Linux to blow away the MBR again, and try this a third time.

If I understand the error messages above correctly, the device driver  
for the SATA drive is detaching the device for some reason, and the  
g_vfs_done() message is simply due to the drive having been yanked.   
I have never seen any messages that indicate why the HDD is being  
detached.

I did not have these problems when I installed from snapshot 011  
(November 2005), but when I built/installed head as of ~3 days ago,  
the above problems surfaced, and after a few crashes I had to  
reinstall from scratch (after overwriting the MBR with GRUB).

Has anyone seen similar behavior?

Does anyone know how to fix this?

Barring answers to the above, can anyone provide pointers in tracking  
this down?

Thanks,
Jason