Formatting and Mounting a Flash Drive
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
15Apr20  Everett Lipman
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Formatting:

1. cat /proc/partitions | grep sd

   Take note of what you see in the output, which may be nothing.
   You will be looking for this to change.

   You can use the 'lsblk' command rather than 'cat /proc/partitions'
   if you prefer.

2. Insert the flash drive into a USB port on your RPi.
   Click on 'Cancel' if you are asked whether you want to
   open the File Manager.

3. cat /proc/partitions | grep sd

   You should now see two new lines for devices sda and
   sda1.  If sda was present when you first did this, your
   flash drive will be assigned a different device name,
   such as sdb.  Here I assume it is sda.  You will see
   one entry for the whole drive (sda), and one for each
   existing partition (sda1, sda2, etc.).

4. df | grep sd

   If you get some output showing your flash drive's device, for example

   /dev/sda1       15126544      16  15126528   1% /media/pi/KINGSTON

   that means the RPi automatically mounted it.  We want to do this
   manually after we have formatted the drive.  If there is no output,
   skip the next step.

5. sudo umount /dev/sda1

   Repeat this for every mounted partition on your flash drive,
   as listed in the output from the previous step.

6. df | grep sd

   There should be no output that corresponds to your
   flash drive this time.

7. sudo fdisk /dev/sda

   Be extremely careful with this command!  You can wipe
   out your SD card or other storage if you type the
   wrong device name or make mistakes below.  If your
   flash drive was not assigned the sda device, be sure
   to use the device it is on!

8. At the Command prompt, type 'p' (without the quotes)

   This should display the present partition table for the drive.
   fdisk does not actually change anything until you write the
   table to disk with the 'w' command.  So you can always quit
   in the middle with 'q' and there will be no changes.

9. Type 'd' to delete any existing partitions, until the 'p'
   command returns an empty partition table.

10. Type 'n' to create a new partition, 'p' to use a primary
    partition, and number the new partition '1'.  Choose the
    default First and Last sectors to use all available space.

11. Type 'a'.  It should make partition 1 bootable.  We will not
    use this now, but if you decide later to install a bootable
    system on this drive, it will be ready.
    
12. Type 'p'

    You should see a single partition that looks like this:

    Device     Boot Start      End  Sectors  Size Id Type
    /dev/sda1  *     2048 30277631 30275584 14.4G 83 Linux

13. Type 'w' to write the partition to disk.   

14. sudo mkfs.ext4 -O ^has_journal /dev/sda1

    The option is a capital letter 'O', not a zero.
    This creates a Linux ext4 filesystem on partition sda1
    with no journaling.  If you are using a traditional
    hard drive (not flash memory), you would leave out
    the '-O ^has_journal' so that the filesystem would
    use journaling.

    There are similar mkfs commands for other filesystems,
    for example mkfs.vfat.   Other filesystems may not
    preserve permissions or other Linux information, however.

15. sudo fsck /dev/sda1

    This checks the new filesystem, and should tell you it is clean.
    Your drive is now formatted, but is not yet mounted.

Please note: formatting the drive wipes out all existing
data, so this is typically done only once, unless you want
to destroy the data and reuse the drive for something else.
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Mounting:

1. df | grep sd

   You should not see any partitions listed that correspond to
   your flash drive (for example, /dev/sda*).

2. sudo mount /dev/sda1 /mnt

3. df | grep sd

   You should now see something like this

   /dev/sda1       14900144   36944  14089928   1% /mnt

   showing that partition 1 on your flash drive (the only
   partition on the drive) is now mounted on /mnt.  This
   means that anything you copy to or change in /mnt will
   be on the flash drive until you unmount the partition.

4. sudo rsync -av /home/pi/ /mnt/pi_13Apr16

   You should substitute today's date for '13Apr16'.
   This command backs up your home directory to the
   specified directory on the flash drive.  Note that
   the trailing slash is necessary immediately following
   '/home/pi'.  You should run backups like this often.

   If you now try to write a file to /mnt without using
   sudo, permission will be denied.

5. ls -alF /mnt

   You will see output that looks like this:

   total 28
   drwxr-xr-x  4 root root  4096 Apr 12 13:27 ./
   drwxr-xr-x 21 root root  4096 Dec 31  1969 ../
   drwx------  2 root root 16384 Apr 12 13:10 lost+found/
   drwxr-xr-x 28 pi   pi    4096 Apr 12 12:30 pi_13Apr16/

   Notice that the directory ./ (which in this case
   corresponds to /mnt) is owned by root, and the
   permissions indicate that others (the rightmost r-x
   column) do not have write permission.

6. sudo chown pi:pi /mnt

7. ls -alF /mnt

   Now you should see this:

   total 28
   drwxr-xr-x  4 pi   pi    4096 Apr 12 13:27 ./
   drwxr-xr-x 21 root root  4096 Dec 31  1969 ../
   drwx------  2 root root 16384 Apr 12 13:10 lost+found/
   drwxr-xr-x 28 pi   pi    4096 Apr 12 12:30 pi_13Apr16/

   Notice that the owner and group for ./ are both pi.
   You should now be able to edit and create files in /mnt
   without using sudo.

8. When you are done using the drive,

   cd

   sudo umount /mnt

   Equivalently, you can run

   sudo umount /dev/sda1

   If you get this error message:

   umount: /mnt: target is busy
           (In some cases useful info about processes that
           use the device is found by lsof(8) or fuser(1).)

   It means that some file in /mnt is being used by a program, or
   some program, such as bash, has its current working directory
   set to /mnt.  Make sure you cd out of /mnt in all of your open
   shell windows and try again.

9. df | grep sd

   There should be no longer be output that corresponds
   to your flash drive.

10. ls -alF /mnt

    You should see something like this:

    total 8
    drwxr-xr-x  2 root root 4096 Mar 18 01:05 ./
    drwxr-xr-x 21 root root 4096 Dec 31  1969 ../

    indicating that the directory is empty.  Once again
    /mnt is owned by root:root (user:group).  This is
    because the flash drive is unmounted and /mnt once
    again refers to the original empty directory in the
    root filesystem on your sd card.  If there were any
    files in /mnt before you mounted your flash drive,
    they should reappear there now.

Once you confirm that your drive is no longer mounted,
it is safe to remove it from the USB socket.  You may
ignore any "Drive was removed without ejecting" message
from the RPi desktop.
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To use the flash drive in the future:

1. Insert the flash drive into a USB port on your RPi.
   Click on 'Cancel' if you are asked whether you want to
   open the File Manager.

2. df | grep sd

   If your drive has been mounted automatically, for
   example if you see

   /dev/sda1       14900144  755780  13371092   6% /media/pi/ab478080-df49-47b4-8a56-c7563b13ab7c
   
   type

   sudo umount /dev/sda1

3. sudo mount /dev/sda1 /mnt 

   or use whatever mount point you choose instead of /mnt.

4. When you are done,

   sync
   sudo umount /dev/sda1