Formatting a Hard Drive with Linux, for Linux

I never thought I would struggle that much with an external Hard Drive (HD) formatting operation under Linux.
But I did, so here is this article, describing step-by-step what one shall do to smoothly format a HD under Linux, using the console only (it means no UI, neither a mouse. sigh)

However don't get too excited, the context of this article is quite narrow.

First of all it sticks to Linux Ext formats, Ext4 in the current use-case. Furthermore the Linux distrib is a light Raspbian (the one used by Raspbmc) on a Raspberry Pi. Finally this focuses on a single partition scheme only. I may explain partitionning  more into details in a further post if needed.

Before we start make sure you've got:

• A system running Linux (a Pi in this example)
• Basic knowledge in Linux bash command lines
• A USB HD that can be safely erased

First of all, open a bash prompt onto the machine which will be in charge of formatting the target HD, e.g. a remote ssh session, a command prompt from the Linux Graphical User Interface (GUI) such as X, or directly the command prompt given by your system like Arch Linux would do.

We'll assume we are logged as root. If you think it's bad practice, then simply prefix the following bash commands with sudo.

Plug your HD in and see how your system sees it:

fdisk -l

This should give you an output like:

root@raspbmc:~# fdisk -l
Disk /dev/mmcblk0: 15.9 GB, 15931539456 bytes
255 heads, 63 sectors/track, 1936 cylinders, total 31116288 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0006e204
        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1               1      143360       71680    b  W95 FAT32
/dev/mmcblk0p2          151552    31115263    15481856   83  Linux

A few explanations about this output:

• The whole chunk between lines 2 and 10 would repeat for each disk detected by your system.
• Line 2 shows the actual Linux device against which your HD has been bound: it is know as mmcblk0. That's what we were looking for actually. We can also see the size of the device, which is almost 16 GB in this example. In this case, it is an SD card (already formatted).
  Let's call this device <myDevice> as a generic placeholder for the name you will find for your own device. However my output sample will still show mmcblk0 for the sake of the example.
• Lines 9 and 10 show the partitions that already exist, if any. Each of them has been bound to its own Linux device, respectively mmcblk0p1 and mmcblk0p2. It may happen if the disk you're using has been pre-formatted already.

We can now run the fdisk utility in interactive mode, i.e. the prompt now belongs to fdisk until you exit it (so no more need to prefix anything with sudo in this section):

fdisk /dev/<myDevice>

You should see something like this:

root@raspbmc:~# fdisk /dev/mmcblk0
Command (m for help):

As you can see, the prompt is not the bash one anymore, but the fdisk one.

Out of curiosity, have a look to the available commands:

m

You should see something like this:

Command (m for help): m
Command action
   a   toggle a bootable flag
   b   edit bsd disklabel
   c   toggle the dos compatibility flag
   d   delete a partition
   l   list known partition types
   m   print this menu
   n   add a new partition
   o   create a new empty DOS partition table
   p   print the partition table
   q   quit without saving changes
   s   create a new empty Sun disklabel
   t   change a partition’s system id
   u   change display/entry units
   v   verify the partition table
   w   write table to disk and exit
   x   extra functionality (experts only)

As we simply want to wipe out all the data on this HD, let's check if any partition exists already so we can get rid of it:

p

You should see something like this:

Command (m for help): p

Disk /dev/mmcblk0: 15.9 GB, 15931539456 bytes
255 heads, 63 sectors/track, 1936 cylinders, total 31116288 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0006e204

        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1               1      143360       71680    b  W95 FAT32
/dev/mmcblk0p2          151552    31115263    15481856   83  Linux

Which is exactly the output we had earlier, when running fdisk in non-interactive mode.
Delete existing partitions if needed:

d

You should see something like this:

Command (m for help): d
Partition number (1-4): 

Simply enter the number corresponding to the line of the partition you want to delete and hit enter. Make sure no partition remains (if needed) by printing the partition list again using the 'p' command.

Now we've wiped out the existing partitions (if any), let's create a new one:

n

You should see something like this:

Command (m for help): n
Partition type:
   p   primary (2 primary, 0 extended, 2 free)
   e   extended

We'll create a primary partition by default. Very roughly, we can only create 4 partitions, that's a hard limit. To work that around without breaking everything, some clever guys introduced the extended ones, which can be used to nest as many logical partitions as needed. This forum post gives a bit more details about this.
Once a partition type has been selected, simply follow the instructions to build your partition. Each prompt comes with a default value which all drive to a partition covering the whole disk space.

Now we've got our new partition, it will be automatically bound to a new device within your /dev folder. Let's see what device has been created (we'll name it <myDevicePartition>) by using the 'p' command again.
It is very important as from now on we are going to deal with the partition we've just created within the HD, not the HD itself. E.g. in this example case, /dev/mmcblk0 points to the HD, while /dev/mmcblk0p1 and /dev/mmcblk0p2 point to the respective existing partitions within the HD.

If you are happy with your work, ask fdisk to write changes and exit:

w

In case you've done rubbish, simply use the 'q' command so you don't save any changes on exit.

We are halfway from having a usable HD now. The disk is partitioned, meaning it has an internal structure able to host a file system (a single one in this example).
Let's format our partition:

mkfs.ext4 /dev/<myDevicePartition> 

In order to access our freshly formatted HD, we have to mount it, i.e. build an access point into the existing file system (the one hosting the bash you have used to partition your HD).

This access point is a simple directory. We'll create it into the /media directory for the sake of Linux good practices (assuming it does exist as it could also be named /mnt or /mount), but it could be any other really:

mkdir /media/<myMountPoint>

Where <myMountPoint> is a name of your choice.

We can now mount the device (which is the -only- partition of your HD)

mount -t ext4 /dev/<myFormatedDevice> /media/<myMountPoint>

Some systems automatically mount the HD they detect either at startup or on-the-fly. In case your system is not that clever, you can manually make this mount permanent.
You need to edit your /etc/fstab configuration file and add the following line:

/dev/<myFormatedDevice> /media/<myMountPoint> ext4 defaults 0 0

Other sources used for this article: http://superuser.com/a/643797 and fdisk help.

That's all folks!