I've bricked my Android smartphone a bit :) I've found how to enter boot menu on startup, but the only possible options are make backup or reset to defaults with complete data loss. Backup to flash made several 2Gb .backup files. There were no exact information about their format. Appeared, that it is single Linux Ext4 divided into several 2Gb blocks. Each block has 512 byte header unnecessary for us. Run unix dd unility (or dd.exe under Windows, or even bbcopy.exe) to reconstruct Ext4 image in single file. Do it on partition supporting large files above 2Gb (NTFS, Linux Ext4, FBSD UFS, etc.)

dd if=userdata_20160101_000020.backup skip=512 bs=128k iflag=nocache,skip_bytes oflag=nocache,append conv=notrunc of=img.ext4
dd if=userdata_20160101_000020.backup1 skip=512 bs=128k iflag=nocache,skip_bytes oflag=nocache,append conv=notrunc of=img.ext4
dd if=userdata_20160101_000020.backup2 skip=512 bs=128k iflag=nocache,skip_bytes oflag=nocache,append conv=notrunc of=img.ext4
dd if=userdata_20160101_000020.backup3 skip=512 bs=128k iflag=nocache,skip_bytes oflag=nocache,append conv=notrunc of=img.ext4
dd if=userdata_20160101_000020.backup4 skip=512 bs=128k iflag=nocache,skip_bytes oflag=nocache,append conv=notrunc of=img.ext4
dd if=userdata_20160101_000020.backup5 skip=512 bs=128k iflag=nocache,skip_bytes oflag=nocache,append conv=notrunc of=img.ext4

Now we can mount final ext4 image under linux (e.g. into /mnt directory) and extract all necessary data

mount -t ext4 -o loop img.ext4 /mnt/

I heard that there are some Windows tools for reading Ext4 images, but I didn't use any of them.

FB