booting Android from SD card
I have been successfully able to boot from SD card on QMobile Z8 by:


Replicating eMMC partition table by partitioning SD card on a Ubuntu machine using parted and fdisk
Flashing factory firmware images to these newly created partitions using dd
Modifying fstab.qcom &amp; init.tegra.rc files in kernel (boot.img) and recovery.fstab and uneventd.rc files in TWRP recovery to initiate mounting and booting from SDCard instead of internal memory


It was successful after some experiments. I think this method should be applicable to any device with similar configuration. However, files to be edited may vary from device to device.  

If you want to put only apps' external data to SD card and not the whole partition, you can edit fstab in boot.img and storage list in framework-res.apk on Android 5 and older.

For more details: [HOW TO] BOOT FROM SD CARD [SUCCESSFULLY] on QMobile Z8 with BRICKED/DEAD eMMC