5.2.1 boot fails; "Could not find Primary Volume Descriptor"

Hi

I am having problems booting FreeBSD 5.2.1-RELEASE on a system I
have. Booting from CD gives me:

----
[other messages from the BIOS]
CD Loader 1.01

Building the boot loader arguments
Read Error: 0x01
Could not find Primary Volume Descriptor
----

Booting from floppies works OK.

The same happens when trying to boot a CD of 5.1-RC1 that I happen to
still have around. 4.9-RELEASE boots fine from CD.

Hardware is an older Dell Pentium Pro system, with a Samsung SM-532
DVD-ROM CD-RW drive.

Any ideas why this happens? I've worked around it, but it's a bit
awkward. I can't find any details of why it happens in searching mailing
lists.

Thanks

Nicolai