Making an Image of a Disk (or Partition) Using dd WITHOUT Including Extra Empty Space

This write-up helped me a lot with this.

I have a Raspberry Pi that gets a lot of use as a media control station, and as a result, it periodically needs to be re-imaged. This is a lot faster and easier than re-installing everything from scratch and then setting up Mythtv, all my network settings, etc. So, a long time ago I used dd in Linux to make an image of a fresh install with all the basic things setup for my network, including Mythtv client. That install was on a microSD card that was 8 GB, so the resulting .img file was 8 GB even though at least half of that was unused. When I tried to write that image to a different 8 GB microSD card, it failed because of insufficient space. If one card had even one bit less (for reasons I do not know), it would be too small. What I needed was to get rid of all that empty space in the .img file.

Write the .img file to a larger memory card that will hold all 8 GB. Then use GParted to shrink the partition that has all the unused space in it to a reasonable size. Now, use sudo fdisk -l -u=sectors to find the number of the last sector used by the last partition.

Disk /dev/sdb: 7.4 GiB, 7960788992 bytes, 15548416 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: 0x00ed4

Device     Boot  Start      End  Sectors  Size Id Type
/dev/sdb1         4096   147455   143360   70M  c W95 FAT32 (LBA)
/dev/sdb2       147456  6348892  6421187  7.4G 83 Linux

 

Make a new .img file from this memory card with the smaller partition. The key here is the parameter "count."

sudo dd if=/dev/mmcblk0 of=NO_WASTED_SPACE.img bs=512 count=6348892

where the count value is the number of the last sector of the last partition you saw in the fdisk listing. That makes dd stop reading anything after the last partition, namely all that extra empty space. Now you have an image file that is just the size you need, it won't take up extra storage space because of the unused space in the image, and when you write the image to a disk (memory card), you won't get errors and failure because the image is larger than the card you are trying to write it on.

 

Something important I learned, the sector size on the block device (memory card) shown via fdisk was 512. When telling it the sector to stop on, you have to make sure the input and output blocksize (bs=) is the same as the sector size, in this case 512. The first time I tried it, the image file was turning out to be huge because I use a block size of 4096 (bs=4096), but the number I used as the last sector to read (count=) was based on a sector size (block size) of 512. When I switched bs back to 512 (I believe the default), it worked fine. Now my image file is 3 GB instead of 8 GB, less space sitting on my hard disk drive, and it won't be picky writing to any device with 3 GB space or more.