Backup your SD cards

It makes sense to backup your SD cards after e.g. successfully setting up you Raspberry Pi. A basic command could look like this:

sudo bash -c "dd if=/dev/sdc bs=1M status=progress | gzip -9 > backup.gz"

If you don’t want to backup the whole disk or your partition is smaller than the SD card, you can also backup only part of the card.

First list the disk geometry:

sudo fdisk -l /dev/sdc

Disk /dev/sdc: 29,28 GiB, 31439454208 bytes, 61405184 sectors
Disk model: STORAGE DEVICE  
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: 0x836a2bae

Device     Boot   Start      End  Sectors  Size Id Type
/dev/sdc1          8192  1056767  1048576  512M  c W95 FAT32 (LBA)
/dev/sdc2       1056768 16416767 15360000  7,3G 83 Linux

We backup in blocks of 1M (1024*1024 bytes), so calculate how many blocks we need to read. Multiply the number of sectors used on the disk by the sector size and divide that by the backup block size:

(16416767+1)*512 / (1024*1024) = 8016 blocks of 1M

and use that number as the block count:

sudo bash -c "dd if=/dev/sdc bs=1M count=8016 status=progress | gzip -9 > backup.gz"