How to dump the SDCard content to an image file

This post shows a way to dump the content of a booting SDCard to an image file with purpose of duplicating the SDCard or sharing its image.

Determine the origin SDCard blocksize and the number of sectors to be copied

$ sudo fdisk -l /dev/sdb
Disk /dev/sdb: 7.3 GiB, 7780433920 bytes, 15196160 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: 0x00000000

Device     Boot Start    End Sectors Size Id Type
/dev/sdb1        2048  34815   32768  16M 53 OnTrack DM6 Aux3
/dev/sdb2  *    34816  67583   32768  16M  c W95 FAT32 (LBA)
/dev/sdb3       67584 190463  122880  60M 83 Linux

The line Units: sectors of 1 * 512 = 512 bytes describes the size of a sector on this SDCard. In this example, it’s 512 bytes.

In this example, the total number of used sectors is it’s 32768 for /dev/sdb1 plus 32768 for /dev/sdb2 plus 122880 for /dev/sdb3, a total of 188416 sectors.

Dump it in an image file

sudo dd if=/dev/sdb of=dump.sdcard bs=512 count=188416

Duplicate the image in another SDCard

sudo dd if=dump.sdcard of=/dev/sdX