DVD writing oddity

[D. Hugh Redelmeier]

I have a shell script that I've been using for years to burn ISO
images.  I just tried it with Fedora 13, and it acted up in an odd
way.

It burnt a DVD+R with this command:
  growisofs -Z /dev/cdrw=/mnt/space-1500/tmp/08mythburn.iso -use-the-force-luke=notray -use-the-force-luke=dao

When that completed (successfully, it turns out), it executed this
command to check the result:
  cmp --bytes 4291420160 /mnt/space-1500/tmp/08mythburn.iso /dev/cdrw

This command failed with this message:
	cmp: /dev/cdrw: No medium found

In other words, the drive didn't seem to have a disk in it, or at
least not one that could be read as a raw bytestream.

The script ejected the disk; I inserted it again; I ran the cmp again
and it worked.

This has worked in the past.  Why does it not work now?