data storage: write acces and writing on wrong partition

[Sibi]

Hi!
I have a problem with owncloud and it's writing behavior.
I changed the datadirectory to "/media/cloud/data" (cloud is my partition I want to use for storage), but owncloud says it does not have the permission to write there. So I changed the permissions to 0770 and changed the ownership to "www-data", this changed nothing.
One time I however was able to fix this (I don't know how) but then owncloud did not write to the other partition. Instead, all files were writen to the "var/usr/owncloud".
I also tried symlink, but that did not work either!
Do you have any suggestions to fix this issue? Thanks!