OS6 - FSTAB to Mount a Second NAS

I do that to have shares on another NAS available to OwnCloud (since it's native external file capbility seems broken).

You first need to create a directory to be the target.  (This may be what you forgot).  Let's say I want it to be "External".  Then you

mkdir /data/External

Now, if you want to mount the "Exported" share on the NAS at 192.168.1.200 in "External", you add this to fstab:

192.168.1.200/data/Exported /data/External nfs rw 0 0

This assumes that you want to export via NFS (which I recommend, since it is a native Linux file system), and so you have to have NFS enabled for the "Exported" share (or share in which "Exported" resides if it's a directory within a share).  Turning on NFS for the share on the ReadyNAS puts the share in the /etc/exports file, so you don't need to do that yourself (in case you are Googling and it tells you to do that).  You can use CIFS instead (lower case in the fstab entry) if there is a reason you don't want to use NFS and CIFS is enabled for the share.  There are some other options as well, but these are typical.

I tried using the NAS name, and it seems to fail sometimes.  Using the IP address works consistently for me as long as the exporting NAS is on when the importing NAS boots.

Note that when you do this, the NAS may show you a "phantom" volume on the volumes page.  Mine didn't until I upgraded to 6.8.x., then it did, but later it went away.  At least I'm fairly certain this was the cause.