bitcoind not using correct directory inspite of datadir argument I configured a non-pruned bitcoin full node on a RaspberryPi 3 using this tutorial - http://raspnode.com/diyBitcoin.html

I am using a 1TB external HDD to store the blockchain. I synced the full blockchain on my laptop and transferred it to this external HDD. 

I am able to mount this external HDD as per the instructions in the above tutorial under the folder ~/bitcoinData and have also setup the automount.

However, even when I run bitcoind using the arguments bitcoind -datadir=/home/pi/bitcoinData -daemon, it still is using the default ~/.bitcoin (on the RPi SD Card) to download the blockchain from scratch even though the full blockchain is available in the mounted drive.

I tried to mount bind the two directories in /etc/fstab/ using /home/pi/bitcoinData    /home/pi/.bitcoin       none    bind    0       0
but still the same issue occurs. Any idea why bitcoind is not using the mounted drive to download the blockchain inspite of the argument datadir=/home/pi/bitcoinData?
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).