Long-time UNIX guy here, but relatively new to the world of Android.

I have recently purchased an Asus MemoPAD (ME103K) ; I then became root, and took a dd image of the read-only system partition to the external SD card:

$ su
# dd if=/dev/block/platform/msm_sdcc.1/by-name/system \
         of=/storage/MicroSD/system.img bs=1M
# ls -l /storage/MicroSD/system.img
-rw-r--r-- 1 root root 2147483648 Sep 27 13:15 system.img

The size (exactly 2GiB) was a bit suspicious - could it be that this was because of the FAT32 partition on the SD card?

No, it was not - tune2fs -l revealed that this was indeed, a valid EXT4 image, exactly sized at 2GiB, which passed fsck -f with no errors at all. And fastboot (from the linux machine attached to the tablet) concurred, after an adb reboot bootloader:

linuxbox# fastboot getvar all
(bootloader)  version-bootloader: 3.03
(bootloader)  version-hardware: rev_c
(bootloader)  variant: LEOPARDCAT 16G
(bootloader)  version-baseband: H00_0.16.F_0521
(bootloader)  serialno: 0a3dXXXX
...
(bootloader)  partition-type:system: ext4
(bootloader)  partition-size:system: 0x0000000080000000

That size, is indeed 2GB:

linuxbox# python2 -c 'print 0x0000000080000000'
2147483648

So, all is good - I have a backup of the image. Now to test restoring it.

I try to flash the system.img back to the tablet - to make sure I can recover from anything, the sort of bullet-proof backup we do in the Unix world (e.g. restore contents of a drive via dd if=backup.image of=/dev/sdXXX).

Everything related to adb and fastboot work flawlessly - so I try...

linux_box# fastboot devices
0a3dXXXX     fastboot

linux_box# mount /dev/sdcard /mnt/sdcard
linux_box# cp /mnt/sdcard/system.img .
linux_box# fastboot flash system system.img
error: cannot load 'system.img'

Hmm. I download and build the android-tools-5.1.1 of my distribution from sources, adding debug information - and step in the debugger, to see this failure:

# gdb --args fastboot flash system system.img
...

Interesting - even though I am in a 64bit machine, apparently there are issues that turn the file size "negative" (in a 32bit world, the file size of my image, 2^31, is indeed considered negative - to be exact, -2147483648.

OK, fine - how do they flash large image files in Android?

Googling, searching - turns out they use this make_ext4fs tool, that creates flashable images. In fact it is part of what I just compiled, so I might as well use it:

# mkdir /system
# mount -o loop,ro system.img /system
# ls -l /system
total 208
drwxr-xr-x 106 root root   8192 Sep 17 22:24 app
drwxr-xr-x   3 root 2000   8192 Sep 26 21:08 bin
-rw-r--r--   1 root root   6847 Sep 12 16:59 build.prop
drwxr-xr-x  19 root root   4096 Sep 26 21:08 etc
drwxr-xr-x   2 root root   4096 Aug 11 22:27 fonts
drwxr-xr-x   4 root root   4096 Sep 12 16:56 framework
drwxr-xr-x  10 root root  16384 Sep 12 16:59 lib
drwxr-xr-x   2 root root   4096 Jan  1  1970 lost+found
drwxr-xr-x   3 root root   4096 Aug 11 22:18 media
drwxr-xr-x  59 root root   4096 Aug 11 22:29 priv-app
-rw-r--r--   1 root root 126951 Aug  1  2008 recovery-from-boot.p
drwxr-xr-x   3 root root   4096 Aug 11 21:02 scripts
drwxr-xr-x   3 root root   4096 Aug 11 21:02 tts
drwxr-xr-x  11 root root   4096 Sep 26 21:08 usr
drwxr-xr-x   8 root 2000   4096 Aug 11 22:29 vendor
drwxr-xr-x   2 root 2000   4096 Sep 26 21:09 xbin
    
# ../extras/source/extras/ext4_utils/make_ext4fs \
      -l 2048M new_system.img /system
Creating filesystem with parameters:
    Size: 2147483648
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 8192
    Inode size: 256
    Journal blocks: 8192
    Label: 
    Blocks: 524288
    Block groups: 16
    Reserved block group size: 127
Created filesystem with 2666/131072 inodes and 375014/524288 blocks

Cool - so I can apparently build system images from plain old folders. The sky will be my limit - I'll be able to add anything I want to this image.

Let's burn it...

# fastboot flash system new_system.img
erasing 'system'...
OKAY [  0.064s]
sending 'system' (2088960 KB)...
^C

I waited for 1h before hitting that Ctrl-C. And had to power-cycle the tablet, which booted back in fastboot mode.

This is not looking good.

What if I build a smaller image? Maybe the 2GB are somehow an issue, and this partition is not used to full capacity - it has free space:

# ../extras/source/extras/ext4_utils/make_ext4fs \
      -l 1536M new_system.img /system

#  ./fastboot flash system system.img 
erasing 'system'...
OKAY [  0.065s]
sending 'system' (1572864 KB)...
OKAY [ 51.039s]
writing 'system'...
OKAY [235.080s]
finished. total time: 286.183s

OK, this looks very promising (and only took 5 min). I guess I can now reboot back and everything should be normal, yes?

No :-)

I don't mind a temporarily bricked device, as long as I do get to control it in the end (machines that I am not a master of, are machines I don't care to operate ;-)

Any ideas on what I did wrong and what I can do to fix this?

Thanks in advance.

P.S. I checked the Asus support page for my tablet - they only provide the sources for the kernel, and the Over-the-air .zip file. That in turn contains a file-system level backup from the root - i.e. the system folder exists in there as just a folder, not an image, not a system.img that I can flash - so that doesn't really help me.