Mount external hard drive with write permissions

This post has been created to keep a track of this very usual issue that I encountered recently. There are many discussions around it already but I thought that adding a very simple article with a few explanations couldn't harm. So here it is.

Linux permissions do not apply to VFAT external drives

When auto-mounted, you may see with a simple ls -la that the mounted partition is not accessible with write permission. No matter how many chmod you do, the result will remain the same : no write access. Don't worry, this is a normal behavior as the FAT32 filesystem do not support linux permissions.

Forget auto-mount

To resolve this issue, you should reconsider it. The permissions do not apply to the partition but they do to the mount point. It means that you'll have to tell linux to grant write permissions at mounting time.

Unmount your disk

umount /dev/sda1

Now mount it back with the following options

mkdir /mnt/mydisk
mount -t vfat /dev/sda1 /mnt/mydisk -o umask=0

It should now work as expected. The key option is umask which is the opposite mask to the one you give to chmod. 0 means that all users have all rights (Note: keeping it like this may be dangerous)

Automatically mount the disk at startup

This is done by editing the /etc/fstab file. Simply add the following line to it

UUID="BF12-2F81" /mnt/mydisk vfat  defaults,uid=me,gid=mylinux,umask=0    0    0

The UUID is the unique identifier of your disk (you can get it by running blkid in a console).
As you can see, I have added uid (user) and gid (group) options to allow one specific user to have write access on my disk.

A few reminders

If you're used to have your disk mounted on  /media and you're wondering why /mnt is often used, here is the explanation :

/media is usually used for auto-mounted disks
/mnt is the one that should be used when manually mounted

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Partition your SD Card for Pi

Create the parition table

1. Plug your SD Card to your computer.

2. Identify your /dev/ node

sudo fdisk -l

The output lists your disks :

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1        2473    19864341    7  HPFS/NTFS
/dev/sda2            2474       19456   136415745    f  W95 Ext'd (LBA)
/dev/sda5            3279        6374    24868588+   7  HPFS/NTFS
/dev/sda6            6375       19456   105081133+   7  HPFS/NTFS
/dev/sda7            2474        3237     6133760   83  Linux
/dev/sda8            3237        3278      330752   82  Linux swap / Solaris

Partition table entries are not in disk order

Disk /dev/sdb: 3974 MB, 3974103040 bytes
4 heads, 16 sectors/track, 121280 cylinders
Units = cylinders of 64 * 512 = 32768 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1         611       19544   83  Linux
/dev/sdb2             612      121280     3861408   83  Linux

Note : sda identifies your hard drives (or partitions) while sdb (or c, d, ...) is used for removable disks

My SD Card is referred as /dev/sdb and it currently has 2 partitions.

3. Clear the partition table

sudo fdisk /dev/sdb/

At the fdisk command line prompt, enter 'o'. It will create a new virtual  DOS partition table :

Building a new DOS disklabel with disk identifier 0xd39a6636.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.

Now enter 'p', it will display the SD Card properties. Note the card size somewhere, we will need it later :

Disk /dev/sdb: 3974 MB, 3974103040 bytes
...

4. Set SD Card geometry

Enter 'x' to go into the expert mode.

The setup configuration is : 255 heads and 63 sectors (512 bytes each). We need to calculate the number of cylinders :

cylinders_nb = card_size_bytes / heads_nb / sectors_nb / sector_size

Here :

cylinders_nb = 3974103040 / 63 / 255 / 512 = 483, 15

The result must be rounded down, which means that cylinders_nb = 483.

Enter 'h' and set the heads number :

Expert command (m for help): h
Number of heads (1-256, default 4): 255

Enter 's' and set the sectors number:

Expert command (m for help): s
Number of sectors (1-63, default 16): 63
Warning: setting sector offset for DOS compatiblity

Enter 'c' and set the cylinders number :

Expert command (m for help): c
Number of cylinders (1-1048576, default 121280): 483

5 . Create a FAT32 partition (boot partition)

Enter 'r' to go back to normal mode.

Follow these steps :

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-483, default 1): 1
Last cylinder, +cylinders or +size{K,M,G} (1-483, default 483): +50

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): c
Changed system type of partition 1 to c (W95 FAT32 (LBA))

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

6. Create a Linux partition (root filesystem)

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 2
First cylinder (52-483, default 52): (press Enter)
Using default value 52
Last cylinder, +cylinders or +size{K,M,G} (52-483, default 483): (press Enter)
Using default value 483

7. Apply the partition table to the card

Enter 'p' and verify that you have these 2 partitions :

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1          51      409626    c  W95 FAT32 (LBA)
/dev/sdb2              52         483     3470040   83  Linux

Enter 'w' to write it on the card.

Format the SD Card

1. Format the DOS partition

sudo mkfs.msdos -F 32 /dev/sdb1 -n DOS

2. Format the Linux partition to ext3

sudo mkfs.ext3 /dev/sdb2 

And that's it ! To avoid doing this manually everytime, the Pi-community have written some scripts that execute these commands for you. One of them can be found here and all you have to do is :

# Make it executable

chmod +x mkcard.txt

# Run it (use your own disk node ! )

./mkcard.txt /dev/sdb

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