umount of /dev failed (BUSY) at shutdown 6.0 Beta1 ISO

I am seeing this message at shutdown:

umount of /dev failed (BUSY)

Everything seems to come up cleanly tho.

I am running a kernel which is GENERIC with the debugging, WITNESS and 
INVARIANTS commented out (tho I believe the umount still fails using 
GENERIC)

The drives are all attached to a Promise TX2000 - but I still get the 
failure using the OSB4 onboard ATA (but I don't want to keep using that 
as it has the well know issue with DMA transfer corruption).

dmesg attached.

Cheers

Mark

Copyright (c) 1992-2005 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
	The Regents of the University of California. All rights reserved.
FreeBSD 6.0-BETA1 #0: Thu Jul 21 08:44:24 NZST 2005
    postgres_at_istral.markir.net:/usr/obj/usr/src/sys/STANDARD
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: Intel Pentium III (996.84-MHz 686-class CPU)
  Origin = "GenuineIntel"  Id = 0x686  Stepping = 6
  Features=0x387fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,PN,MMX,FXSR,SSE>
real memory  = 2147483648 (2048 MB)
avail memory = 2096631808 (1999 MB)
MPTable: <AMI      CNB30LE     >
FreeBSD/SMP: Multiprocessor System Detected: 2 CPUs
 cpu0 (BSP): APIC ID:  0
 cpu1 (AP): APIC ID:  1
ioapic0: Assuming intbase of 0
ioapic1: Assuming intbase of 16
ioapic0 <Version 1.1> irqs 0-15 on motherboard
ioapic1 <Version 1.1> irqs 16-31 on motherboard
npx0: [FAST]
npx0: <math processor> on motherboard
npx0: INT 16 interface
cpu0 on motherboard
cpu1 on motherboard
pcib0: <MPTable Host-PCI bridge> pcibus 0 on motherboard
pci0: <PCI bus> on pcib0
pci0: <display, VGA> at device 1.0 (no driver attached)
fxp0: <Intel 82559 Pro/100 Ethernet> port 0xd400-0xd43f mem 0xfeafe000-0xfeafefff,0xfe900000-0xfe9fffff irq 20 at device 4.0 on pci0
miibus0: <MII bus> on fxp0
inphy0: <i82555 10/100 media interface> on miibus0
inphy0:  10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
fxp0: Ethernet address: 00:e0:81:02:4c:6a
fxp1: <Intel 82559 Pro/100 Ethernet> port 0xd000-0xd03f mem 0xfeafd000-0xfeafdfff,0xfe700000-0xfe7fffff irq 21 at device 5.0 on pci0
miibus1: <MII bus> on fxp1
inphy1: <i82555 10/100 media interface> on miibus1
inphy1:  10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
fxp1: Ethernet address: 00:e0:81:02:4c:6b
isab0: <PCI-ISA bridge> port 0x580-0x58f at device 15.0 on pci0
isa0: <ISA bus> on isab0
atapci0: <ServerWorks ROSB4 UDMA33 controller> port 0x1f0-0x1f7,0x3f6,0x170-0x177,0x376,0xffa0-0xffaf at device 15.1 on pci0
ata0: <ATA channel 0> on atapci0
ata1: <ATA channel 1> on atapci0
ohci0: <OHCI (generic) USB controller> mem 0xfffff000-0xffffffff at device 15.2 on pci0
pcib0: unable to route slot 15 INTA
ohci0: Could not allocate irq
device_attach: ohci0 attach returned 6
pcib1: <MPTable Host-PCI bridge> pcibus 1 on motherboard
pci1: <PCI bus> on pcib1
atapci1: <Promise PDC20271 UDMA133 controller> port 0xefe0-0xefe7,0xefac-0xefaf,0xefa0-0xefa7,0xefa8-0xefab,0xef90-0xef9f mem 0xfebf0000-0xfebfffff irq 27 at device 2.0 on pci1
ata2: <ATA channel 0> on atapci1
ata3: <ATA channel 1> on atapci1
pmtimer0 on isa0
orm0: <ISA Option ROMs> at iomem 0xc0000-0xc7fff,0xc8000-0xd17ff,0xd1800-0xd27ff,0xd2800-0xd37ff on isa0
atkbdc0: <Keyboard controller (i8042)> at port 0x60,0x64 on isa0
atkbd0: <AT Keyboard> irq 1 on atkbdc0
kbd0 at atkbd0
atkbd0: [GIANT-LOCKED]
fdc0: <Enhanced floppy controller> at port 0x3f0-0x3f5,0x3f7 irq 6 drq 2 on isa0
fdc0: [FAST]
ppc0: parallel port not found.
sc0: <System console> at flags 0x100 on isa0
sc0: VGA <16 virtual consoles, flags=0x300>
sio0 at port 0x3f8-0x3ff irq 4 flags 0x10 on isa0
sio0: type 16550A
sio1 at port 0x2f8-0x2ff irq 3 on isa0
sio1: type 16550A
vga0: <Generic ISA VGA> at port 0x3c0-0x3df iomem 0xa0000-0xbffff on isa0
unknown: <PNP0c01> can't assign resources (memory)
unknown: <PNP0303> can't assign resources (port)
unknown: <PNP0c02> can't assign resources (port)
unknown: <PNP0501> can't assign resources (port)
unknown: <PNP0501> can't assign resources (port)
unknown: <PNP0700> can't assign resources (port)
Timecounters tick every 1.000 msec
acd0: CDRW <CREATIVE CD-RW RW8439E/1R09> at ata0-master PIO4
ad4: 38166MB <Seagate ST340014A 3.06> at ata2-master UDMA100
ad6: 39205MB <Maxtor 6E040L0 NAR61590> at ata3-master UDMA133
ad7: 39205MB <Maxtor 6E040L0 NAR61590> at ata3-slave UDMA133
ATA PseudoRAID loaded
ar0: 38166MB <Promise Fasttrak SPAN> status: READY
ar0: disk0 READY using ad4 at ata2-master
ar1: 78411MB <Promise Fasttrak RAID0 (stripe 256 KB)> status: READY
ar1: disk0 READY using ad6 at ata3-master
ar1: disk1 READY using ad7 at ata3-slave
SMP: AP CPU #1 Launched!
Trying to mount root from ufs:/dev/ar0s1a