New internal Burner does not write on DVD-RAM as it is supposed to

Hi everybody,

yesterday I built a BluRay burner into my Mac Pro 5,1 (Lion 10.7.2): HP BD340i

The burner also supports officially writing of DVD-RAMs.

Everything works just fine. Except the DVD-RAMs. I can mount them, I can read them, but nothing can be written to them. As soon as the system tries to write to the disc, an error message pops up "Unknown error -50".

It also impossible to unmount the disk then. I have to force it to get the disc out.

With disk utility you can start to format a disc, but then it also quits the formatting process with the message "System could not write into the last block."

I still have an external DVD burner (Pioneer connected via FireWrie) that supports DVD-RAMs and that I used before. The DVD-RAMs work just fine with it as they did before.

With the drive "wired" to a Windows 7 virtual machine I was able to create folders on the disc, but no files could be written.

Tried everything with no success. Does anybody have an idea how to fix the problem? I bought the burner because I wanted an internal DVD-RAM drive PLUS a BluRay burner. Now I am stil stuck with my external drive that I wanted to get rid of. Is it possible that just the burning of DVD-RAMs is kind of broken in the drive?

Thanks in advance.