Boot hangs after device detection

[Matt]

A few days ago I installed FreeBSD 5.3 on my system with the following specs:

Asus A7N8X-Deluxe Motherboard
AMD Athlon XP3000+
1GB PC3200 RAM
GF4 Ti4200 128MB
60GB IDE Seagate Barracuda
200GB IDE Western Digital
40GB IDE IBM Deskstar
NEC DVD-RW

All went well, I installed the operating system on the 60GB drive and made
the 200GB my home partition. I installed Gnome etc. Then I came to replace
the 40GB drive which contained backups with a DVD-ROM drive. This was my
first reboot of the system since install. I shut it down and replaced the
hard disk with the DVD-ROM and booted up. At the boot loader I chose the
default option and it booted as far as the following lines and then halted
with a complete lock up.

acd0: DVDROM <LITEON DVD-ROM LTD163D/GHR3> at ata1-master UDMA33
acd1: DVDR <NEC DVD RW ND-1300A/1.0A> at ata1-slave UDMA33

A verbose boot halted with the following two lines:

GEOM: Configure ad1s1c, start 0 length 200047002624 end 200047002623
GEOM: Configure ad1s1d, start 0 length 200047002624 end 200047002623

I origionally thought it was a problem with my SiL 3112A SATA chipset
(known to be problematic) so I disabled that with a motherboard jumper and
it made no difference.

Then I considered a problem with one of my devices so I unplugged
everything except the 60GB disk which contains the FreeBSD operating
system itself. I also tried several combinations of different devices and
the system always hangs right after the detection of devices or after the
GEOM: Configure in verbose mode.

I've tried booting in safe mode, non-acpi and verbose modes and have
exactly the same problem in all three.

One odd thing I have noticed is that in the boot prompt if I do lsdev it
only shows the DVD drives if they have a disk in - I assume this is
correct?

Any suggestions on what I can try next will be greatly appreciated. Any
more information you need I will try and provide.