Silent errors when reading CDs

[Jean-Sebastien Roy]

Hi !

(Following an advice from someone at freebsd-questions@, this is a 
repost of a question asked at freebsd-questions@)

I'm currently using FreeBSD 4.10 on an HP D530 SFF.
The system is perfectly stable except for the following problem
I'm unable to understand :

When I mount a cdrom (mount /cdrom), then calculate the MD5 hash
of a big file on a CD (md5 /cdrom/bigfile), the results are often random:
unmounting, mounting again and calculating again the MD5 often result in
a different value. What disturb me the most is that absolutely no errors
are reported in any log (no read errors for example).

I thought the CDROM reader, a LITE-ON LTR-48327S PQS3, was the culprit,
so I replaced it with a PLEXTOR DVDR PX-712A and got the exact same 
results (i.e. random MD5 values). I checked the RAM using memtest and
got no errors. The problem does not occur for files on the harddisk.

hw.ata.atapi_dma is set since both drives support it and it seems to be
required for proper CD/DVD burning. The CD drive is the master on its
own ATA bus.

While the problem occurs on multiple CDs (mostly RW), to my surprise, I 
was not able to reproduce the problem by reading big files on DVDs.

Could someone provide me a hint on what to check next or how to fix this
problem ? Can errors on CDs generate such a behavior ?

Thank you very much in advance,

Regards,

Jean-Sebastien