Mounting a DVD (busy message)

[David Wright <deblis@lionunicorn.co.uk>]

When I mount a DVD on this wheezy desktop, I get:

$ mount /media/cdrom0
mount: block device /dev/sr0 is write-protected, mounting read-only
mount: /dev/sr0 already mounted or /media/cdrom0 busy
$ mount /media/cdrom0
mount: block device /dev/sr0 is write-protected, mounting read-only
$

It doesn't matter how long I wait after inserting the DVD. It seems to
happen whenever I insert a disk.

Otherwise, everything is normal. Unmounting and mounting again
(without ejecting in between) is normal.

/etc/fstab includes the line:
/dev/sr0 /media/cdrom0 udf,iso9660 user,noauto

I can live with this, but I'm curious to know why.

Cheers,
David.