DVD-RW cannot be formatted or blanked

[Markus Huber]

Dear people,

I try to blank resp. format a DVD-RW with following command and 
following output.

[markus at fc4 ~]$ dvd+rw-format /dev/cdrom
* DVD\uffffRW/-RAM format utility by <appro at fy.chalmers.se>, version 4.10.
Error trying to open /dev/cdrom exclusively ... retrying in 1 second.
Error trying to open /dev/cdrom exclusively ... retrying in 1 second.

Same error I get when I use:
$ cdrecord blank

What am I doing wrong? It is the first time I use a DVD-RW on 
Fedora/Linux/at all, and Google is not my friend in this case.

Thank you for help
Markus Huber