Can I place the datastore-folder anywhere I would want? For example on a shared file system? So I would point it to /nfs/forum/datastore/ for example. 

It seems I can configure this in constants.php, but the installer seems to require the datastore folder to be writable in the community root, so I'm wondering if that's just the setup script being stubborn or whether there are other areas that doesn't use the value from init.php / constants.php and expects this to be a writable folder in the forum root either way. 

Edited December 16, 201410 yr by TSP

You can define it in Advanced Configuration in ACP. It will give you a new constants.php to use.