freebsd stable - Jul 2005 - 5.4-RELEASE-p4: shutdown hangs after rebuiding gmirror

Hi. I am running FreeBSD 5.4-RELEASE-p4 on a sun V20Z. It has two seagate 
ST373307LC disks. The disks are mirrored with gmirror:

         Name    Status  Components
mirror/raid1  COMPLETE  da0
                         da1

I unplugged one of the disks, then rebooted to see if it would boot from 
just the one disk. Of course there was no problem there.

I then inserted the other disk back in, followed by

  camcontrol rescan all
  gmirror forget raid1
  gmirror insert raid1 da1

After a while the mirror was OK again. I then rebooted for a second time. 
But the machine would not come up. The last lines printed on the console 
were

Syncing disks, vnodes remaining...1 1 0 1 1 0 0 0 done
No buffers busy after final sync
Uptime: 55m39s
GEOM_MIRROR: Device raid1: provider mirror/raid1 destroyed.
GEOM_MIRROR: Device raid1 destroyed.

So it was stuck somewhere in the shutdown sequence between the gmirror 
destruction and the "Shutting down ACPI" bit. Sending a break to the 
console did not produce a DDB prompt, whereas during normal operation it 
would.

I had to power-cycle to get the boot to work again.

If I leave the disks alone, rebooting does not hang the machine.

This is a bit of a nuisance. Who has any idea where to look to 
troubleshoot this?

Cheers
Michiel

[recap: machine hangs on shutdown - see original message for more
details]
> Syncing disks, vnodes remaining...1 1 0 1 1 0 0 0 done
> No buffers busy after final sync
> Uptime: 55m39s
> GEOM_MIRROR: Device raid1: provider mirror/raid1 destroyed.
> GEOM_MIRROR: Device raid1 destroyed.
FWIW the hangs occur regardless of whether I rebuild a mirror. The problem 
is that the box sometimes hangs and sometimes not. Of course this makes 
the whole gmirror totally useless (as opposed to a bit of a nuisance)

It appears that the hang occurs somewhere between destruction of raid1 and 
'raid1.sync'. (whatever that is)

Does anyone have any clue as to what is going on or how I can debug this 
further? Please don't make me put solaris on this box. :)

Cheers
Michiel