I did everything according to these instructions on the OwnCloud forum
but I get the error:

Data directory (/mnt/usbdrive/owncloud/) is readable for other users

Please change the permissions to 0770 so that the directory cannot be listed by other users.

I gave the permissions like:

 chmod 0770 /mnt/usbdrive/owncloud/

and I changed my /etc/fstab

I used:

chown -R www-data:www-data /mnt/usbdrive/owncloud/

as well. But I still get that error when going to owncloud

This is what my /etc/fstab looks like:

[

EDIT: This error only comes up when I change the data directory. If I keep it the same, I can acces it normally.