MD5 failing on optical media

[Andrew Smallshaw <andrews%sdf.lonestar.org@localhost>]

I've been encountering odd errors whenever I attempt to verify
files on optical discs.  Right now this is on a couple of DVD-RAM
discs but I've encoutered similar issues on +R and +RW discs in
the past.  Before now I've put it down to real errors and defective
media but I've caught it in the act now.  

If I have data and an MD5 checksum file on an optical disc and
attempt to verify it in place I get errors even immediately after
making the checksums.  However, if I copy the files onto my hard
drive first and check those copies against the copied checksum file
everything is successful:

/mnt/udf
n1c$ md5 * >/tmp/MD5; mv /tmp/MD5 .
/mnt/udf
n1c$ cd .. ; umount udf ; mount udf ; cd udf
/mnt/udf
n1c$ md5 -c MD5                                                                 
                           
(MD5) Open-License-Software-Supplement.iso: FAILED
(MD5) Vision-2000.iso: FAILED
/mnt/udf
n1c$ cd /tmp
/tmp
n1c$ mkdir UW7
/tmp
n1c$ cd UW7/                                                                   
/tmp/UW7
n1c$ cp /mnt/udf/* .
/tmp/UW7
n1c$ md5 -c MD5                                                                
/tmp/UW7
n1c$ 

Does anyone have any ideas what may be going on here?  FWIW cmp
doesnt seem to have any issues comparing the disc files against
either the originals or the copies.  System is running 6.0/amd64
RELEASE.  dmesg|grep cd0 reports:

cd0 at atapibus0 drive 0: <TSSTcorp CDDVDW SH-S223C, R4136GBZ996738, SB04> 
cdrom removable
cd0: 32-bit data port
cd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
cd0(viaide1:1:0): using PIO mode 4, Ultra-DMA mode 5 (Ultra/100) (using DMA)

-- 
Andrew Smallshaw
andrews%sdf.lonestar.org@localhost