weird dvd burning issue

[andy baxter]

hello,

I have just encountered a weird issue when trying to burn an iso image 
of a data dvd. I used k3b to do the burn, at 2x speed to an external usb 
dvd burner. In k3b, the verification phase of the burn failed. Also, 
when I md5sum /dev/scd0 it returns a different sum from md5summing the 
iso image.

However, when I did:

root at monkey:~# diff -qr /mnt /media/cdrom0 | tee iso.diff

there was no output and iso.diff was empty, so there is no difference in 
the files on the dvd. I then tried copying /dev/scd0 back to the hard 
disk using:

root at monkey:~# dd if=/dev/scd0 of=/home/andy/Desktop/64studio-from-disk.iso
1633856+0 records in
1633856+0 records out
836534272 bytes (837 MB) copied, 107.058 s, 7.8 MB/s

When I list the two files together, the one copied back from the dvd is 
slightly longer than the original iso:

root at monkey:~# ls -l /home/andy/Desktop/64*
-rw-r--r-- 1 andy andy 836503552 2008-08-13 02:40 
/home/andy/Desktop/64studio_2.1_i386.iso
-rw-r--r-- 1 root root 836534272 2008-08-13 07:38 
/home/andy/Desktop/64studio-from-disk.iso

('64studio-from-disk.iso' is the one copied back by me; the other is the 
original).

Does anyone know what is going on here, and whether this is a known 
issue? My dvd drive reports itself as a 'freecom optical disk drive' in 
lsusb.

andy