Is it necessary to mount an external hard disk to save blockchain to external hard disk on Raspberry PI? I have a Raspberry Pi 3.  I'm running bitcoind for the first time and would like to save the blockchain to an external hard disk.

When I run 
bitcoind -datadir=/media/pi/FreeAgent\ GoFlex\Drive

I get the following error:

Error: Cannot obtain a lock on data directory /media/pi/FreeAgent GoFlex Drive
Bitcoin Core is probably already running. No such file or directory.


I tried stopping the process with

bitcoin-cli stop


But that didn't seem to do anything.  Is the problem that the external hard disk has to be formatted and mounted, as per instructions here?

As a test, I tried writing a file to the hard disk from the system and received the following error:

mv: inter-device move failed: 'testfile' to '/media/pi/FreeAgent GoFlex Drive/testfile'; unable to remove target: Read-only file system


This does seem to confirm this hypothesis that the disk needs to be formatted/mounted.
I get the following error:

Error: Cannot obtain a lock on data directory /media/pi/FreeAgent GoFlex Drive
Bitcoin Core is probably already running. No such file or directory.

  
  I tried stopping the process with

bitcoin-cli stop



Bitcoin Core is already stopped when you get that error. It can't run without the datadir.


  But that didn't seem to do anything.  Is the problem that the external
  hard disk has to be formatted and mounted, as per instructions
  [here][1]?
  
  As a test, I tried writing a file to the hard disk from the system and
  received the following error:

mv: inter-device move failed: 'testfile' to '/media/pi/FreeAgent GoFlex Drive/testfile'; unable to remove target: Read-only file system

  
  This does seem to confirm this hypothesis that the disk needs to be
  formatted/mounted.


No, it does not necessarily need to be formatted.

The problem is that your user does not have the correct permissions to write to your external drive. Make sure that your drive is writable and that your user has the permissions to write to the external drive. You may have to change how it is being mounted to the system (drives always have to be mounted in order to be accessible).