Using DVD-RAM (MATSHITA DVD-RAM UJ-845S)?

Hello,
I upgraded my PowerBook with the MATSHITA DVD-RAM UJ-845S to use also DVD-RAM. I bought an empty DVD-RAM and wanted to use it, but I can not store anything on this disk. It is recogniced in the system:
Firmware Revision: D100
Interconnect: ATAPI
Burn Support: Yes (Vendor Supported)
Profile Path: /Library/DiscRecording/DeviceProfiles/PatchBurn-matshitadvdramuj845s.drprofile
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RAM, -RW, +R, +RW, +R DL
Burn Underrun Protection CD: Yes
Burn Underrun Protection DVD: Yes
Write Strategies: CD-TAO, CD-SAO, DVD-DAO
Media:
Media Type: DVD-RAM
Blank: Yes
Erasable: No
Overwritable: Yes
Appendable: Yes

In Finder I saw that I only have "read" access on this disk.
How is it possible to read & write on the DVD-RAM?

PowerBook G4 1.66GHz Mac OS X (10.4.3)