I'm trying to shrink a raspbian image following this procedure:
The procedure works fine and I am able to get the shrunken image to boot correctly in a raspberry pi.
However, there is a step in the procedure where we improve the compression of the image by filling empty space with zeroes. This requires mounting the image on the filesystem like so.
sudo losetup /dev/loop0 imagename.img -o $((START*512)) sudo mkdir -p /mnt/imageroot sudo mount /dev/loop0 /mnt/imageroot
I am receiving a strange error when I try to mount the 2nd volume on the image to the file system. Despite my research, I have not yet discovered why I am getting this error or how to work around it.
Step-by-step, I first used fdisk to discover where the 2nd partition starts like so:
$ sudo fdisk -l raspbian-stretch-opencv-2.4.img Disk raspbian-stretch-opencv-2.4.img: 9.9 GiB, 10593763840 bytes, 20690945 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: 0xe5723e2e Device Boot Start End Sectors Size Id Type raspbian-stretch-opencv-2.4.img1 8192 93814 85623 41.8M c W95 FAT32 (LBA) raspbian-stretch-opencv-2.4.img2 94208 20785151 20690944 9.9G 83 Linux sryan@ubuntu-artful:~/images$
Then, I attached it to the loopback device like so:
$ sudo losetup /dev/loop0 raspbian-stretch-opencv-2.4.img -o $((94208*512)) $ sudo losetup -f /dev/loop1
However, when I try to mount, I get this rather generic error:
$ sudo mount /dev/loop0 /mnt/imageroot mount: /mnt/imageroot: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error.
I have done surgery on this image by (1) using gparted to shrink the file system, (2) fdisk to change the partition table and (3) truncate to to remove the unused portion of the file, as specified by the procedure. The image itself appears to be okay insofar as I burned it to a microSD card and the raspberry pi booted successfully.
I am mystified as to why I cannot mount the file system following the recommended procedure.
I am doing all of this inside of a virtualbox (5.2.6) virtual machine running ubuntu (17.10.1) where the host os is mac os (10.13.3 Beta). The image file itself resides on an external hard drive which is accessed by the vm via a shared folder with the host os.
One suggestion was to extract the file system from the 2nd partition to a new image and then mount that directly. This would allow mounting without specifying an offset. This seems a little extreme.