Tvheadend

Is the WD box a DAS or NAS? If it's USB-connected, I presume it's auto-mounting into /dev/usb or something similar? I've found in the past that USB devices can mount with root-only permissions by default, which is hardly useful, so check that. If it's an NFS mount, check the permissions anyway, you may not be mounting it rw.

Other thoughts - create a symlink from a local directory to the WD device, see if tvheadend prefers that (no reason it would).

Or do what I do - I record locally because I don't want any latency to corrupt the recording, but then simply copy the file over to my NAS with a post-processing script. Okay, you don't have the recordings immediately appear in XBMC's Recordings menu, but it's hardly awful to set up a shortcut in the Files view to get to them.

Thank you! Ok, I guess you know the problem....
The WD BOX is a true NAS, connected at a gigabyte switch. My recording path is (should be) in the normal Public directory. All users can write at this directory. I've already tried to use symlink. Every program will use this symlink well but not Tvheadend! Your last suggestion to record locally and the copy the file to the NAS works but it is IMHO a workaround not a solution.

The NAS ist mounted this way:

mount -t cifs -o username=XXXX,password=XXXX,uid=1000,gid=1000 //IP.OF.THE.NAS/Public/Shared\ Videos/Fernsehaufzeichnungen /media/Videorecorder

The symlink is in the home directory and is linked to /media/Videorecorder

Is anything wrong with the mount statement? Kaffeine for example writes the streams directly into this directory. What is the reason, why Tvheadend can't do it in the same way?

I would guess that the tvheadend process (which will be running as hts/video more than likely) doesn't have write permission to that CIFS export. When you run Kaffeine, you're running it under your own UID, so it can write to the NAS drive since you can write to it.

I don't know much about CIFS exports (I export from my NAS as NFS) but see if you can export it to more than one user - you may need to mount it a second time with a different username/password, or perhaps use some kind of group membership.

Problem solved! It is really up to the rights that are set on the mountpoint for the CIFS network drive. With file_mode=0666 and dir_mode=0777 every user is allowed to write to the mountpoint - also the "user" Tvheadend. I don't need a symlink but i can use it. Now I can try out, if there are any problems with latency of the NAS. Thank you Prof Yaffle!