Prepare SD card

• Insert SD card / usb reader
• Identify the device:

  MMC card reader

  dmesg
  ...
  Oct 20 08:48:20 tableta kernel: mmc0: new high speed SDHC card at address aaaa
  Oct 20 08:48:20 tableta kernel: mmcblk0: mmc0:aaaa SL08G 7.40 GiB 
  Oct 20 08:48:20 tableta kernel:  mmcblk0: p1
  

  USB card reader

  dmesg
  ...
  Oct 20 08:52:38 tableta kernel: scsi 24:0:0:0: Direct-Access     Generic- SD/MMC           1.00 PQ: 0 ANSI: 0 CCS
  Oct 20 08:52:38 tableta kernel: sd 24:0:0:0: Attached scsi generic sg1 type 0
  Oct 20 08:52:39 tableta kernel: sd 24:0:0:0: [sdb] 32372736 512-byte logical blocks: (16.5 GB/15.4 GiB)
  ...
  Oct 20 08:52:39 tableta kernel:  sdb: sdb1
  

  Let's check once more in /proc/partitions

  cat /proc/partitions
  major minor  #blocks  name
  
  ...
   179        0    7761920 mmcblk0
   179        1    7757824 mmcblk0p1
     8       16   16186368 sdb
     8       17   16185344 sdb1
  

  Here /dev/sdb represents the whole card in the USB reader and /dev/sdb1 is the first (and only) partition on that card. Similarly /dev/mmcblk0 represents the card in the MMC reader and /dev/mmcblk0p1 it's first partions.

  Let's use /dev/mmcblk0 and /dev/mmcblk0pX in the following examples.

  Note: depending on how paranoid your distro is you might not be able run dmesg and read /proc/partitions as normal user.

• Umount any partitions that got automatically mounted (depends on your distro, desktop environment and settings...)
• Partition the card to contain:
  • A small FAT16 partition for boot files (say 128MB, though 32MB or even 16MB should be enough).
  • Partition for the root filessytem (ext4 or similar)

  Following commands will have to be executed under root:

  1. Erase the partition table / beginning of the card:

     dd if=/dev/zero of=/dev/mmcblk0 bs=1M count=1
     1+0 records in
     1+0 records out
     1048576 bytes (1.0 MB) copied, 0.0827764 s, 12.7 MB/s
     

  2. Run fdisk to re-create the partition table and create the partitions:
     • Use the help (m) command list available fdisk commands
     • Create the first partition, set its type to e(W95 FAT16), and set it as bootable (a, needed only for Beagle Bone, but doesn't hurt...)
     • Create the second partition and set its type to 83 (Linux)

     fdisk /dev/mmcblk0
     
     Welcome to fdisk (util-linux 2.25.1).
     Changes will remain in memory only, until you decide to write them.
     Be careful before using the write command.
     
     Device does not contain a recognized partition table.
     Created a new DOS disklabel with disk identifier 0x0c677597.
     
     Command (m for help): n
     Partition type
        p   primary (0 primary, 0 extended, 4 free)
        e   extended (container for logical partitions)
     Select (default p): p
     Partition number (1-4, default 1): 1
     First sector (2048-15523839, default 2048):
     Last sector, +sectors or +size{K,M,G,T,P} (2048-15523839, default 15523839): +128M
     
     Created a new partition 1 of type 'Linux' and of size 128 MiB.
     
     Command (m for help): t
     Selected partition 1
     Hex code (type L to list all codes): e
     If you have created or modified any DOS 6.x partitions, please see the fdisk documentation for additional information.
     Changed type of partition 'Linux' to 'W95 FAT16 (LBA)'.
     
     Command (m for help): n
     Partition type
        p   primary (1 primary, 0 extended, 3 free)
        e   extended (container for logical partitions)
     Select (default p):
     Partition number (2-4, default 2):
     First sector (264192-15523839, default 264192):
     Last sector, +sectors or +size{K,M,G,T,P} (264192-15523839, default 15523839):
     
     Created a new partition 2 of type 'Linux' and of size 7.3 GiB.
     
     Command (m for help): a
     Partition number (1,2, default 2): 1
     
     The bootable flag on partition 1 is enabled now.
     
     Command (m for help): p
     Disk /dev/mmcblk0: 7.4 GiB, 7948206080 bytes, 15523840 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
     Disklabel type: dos
     Disk identifier: 0x549005c5
     
     Device         Boot  Start      End  Sectors  Size Id Type
     /dev/mmcblk0p1 *      2048   264191   262144  128M  e W95 FAT16 (LBA)
     /dev/mmcblk0p2      264192 15523839 15259648  7.3G 83 Linux
     
     Command (m for help): w
     The partition table has been altered.
     Calling ioctl() to re-read partition table.
     Syncing disks.
     

     Note: you can use also cfdisk for a curses-based interface.

• Format the first partition as FAT16:

  mkfs.vfat  -F 16 -n boot /dev/mmcblk0p1
  

• Format the seconds partition as ext4 (or similar):

  mkfs.ext4 -L rootfs /dev/mmcblk0p2
  

• Mount the partitions as/when needed (or just remove and re-insert the card if you have automount enabled).