CD problem on iMac 333

Hi friends,

We have a strange problem with an iMac 333 here, it doesn't want to
read self-burned data CDs (and wants to format them, like macs used to
do long ago with IBM floppies). This happens with CDs I burned and CDs
I got from others; Re-writable CD, burned on the same machine, work
perfectly! I tried the original system 8 the machine was shipped with,
and my good old sys 9.1 I work with on my other macs, but nothing
seems to help. What's going on? Do I miss some extension? What can I
update?

Thanks

Eyal