bitcoind not using correct directory inspite of datadir argument
That tutorial sucks. 

Better seek generic instructions on how to mount external HDD to user's home directory. 

Say you're Joe, you'd use sudo to add a user called bitcoin. 

Then mount /dev/hda1 (or whatever it is) to /home/bitcoin, configure /etc/fstab, reboot, log in as Joe and make sure it works (df).

Then login as bitcoin user, make sure you're in /home/bitcoin (where you should be), download (or just install) Bitcoin Core, and start it. You don't need any config files (but you can add rpcusername and rpcpassword), as by default everything will be stored under /home/bitcoin/.bitcoin/. 

Joe now can use a wallet that connects to Bitcoin Core (executed by bitcoin user).