Problem mounting root from CD9660 on 4.x

[Peter Jeremy]

I have a 4.10 system that I'm trying to build a bootable DVD for (for
disaster recovery purposes).  Whilst it boots OK, it won't mount root
from the native ATAPI device but only off the SCSI emulation.  Has
anyone else seen this?

The ISO image is built with
mkisofs -b boot/cdboot -no-emul-boot -r -J -V MY_RELENG_4_10 -o foo.iso /home/root

The relevant parts of the dmesg are:
atapci0: <VIA 8233 ATA133 controller> port 0xfc00-0xfc0f at device 17.1 on pci0
ata0: at 0x1f0 irq 14 on atapci0
ata1: at 0x170 irq 15 on atapci0
...
acd0: DVD-R <SONY DVD RW DRU-700A> at ata1-master UDMA33
cd0 at ata1 bus 0 target 0 lun 0
cd0: <SONY DVD RW DRU-700A VY03> Removable CD-ROM SCSI-0 device 
cd0: 33.000MB/s transfers
cd0: cd present [2295104 x 2048 byte records]

All the following root specifications failed with ENXIO:
cd9660:/dev/acd0c
cd9660:acd0
cd9660:acd0c

but cd9660:cd0c worked.

-- 
Peter Jeremy