[Sprint Samsung Galaxy S III] DrGit:
This thread is dedicated to booting Qualcomm Snapdragon-based Samsung Galaxy S3 models from an SD card.

As I understand it, this only works because the Snapdragon boot ROM (NOT the bootloader, which resides on the eMMC - this is part of the CPU) has a fallback mode which reads the bootloader from an SD card if the eMMC fails. To date, the only devices I am aware of that have this fallback mode are the Snapdragon Galaxy S3 models designed for use in the US. As such, this will not work on international/Exynos based phones.

The process of preparing an SD card for this is fairly simple. All that is needed is a working, rooted phone and an empty SD card.

The dumps at the end of this post were obtained by various people using a variant of the following command:

busybox dd if=/dev/block/mmcblk0 of=/sdcard/backup.bin bs=1M count=70

Depending on the device, you may need more than 70MB, but on the Sprint Galaxy S3, the first 70MB of the eMMC contains everything needed for download mode without including identifying information in the dump, like the EFS partition for example.

If you are absolutely certain your SD card is empty, you can change the dd command to "of=/dev/block/mmblk1" to write the eMMC backup image directly to the SD card instead of to a file. Be very careful working with dd! mmcblk0 is the eMMC and mmcblk1 is the external SD card.

This should be obvious, but cross-device dumps will not work! This is the main reason so many people are finding themselves in need of this guide!

Use common sense - do your research and don't cross-flash, especially between US and international ROMs!

If your model is not listed there, search the forums or ask someone to make a dump for you.

Please note: if your phone model is not listed here, that doesn't mean this method won't work - however, I would appreciate it if everyone would do their research instead of bombarding me with private messages.

I hope this helps! Flash wisely!

For posterity, this is the content of the OP that led me to discover the SD boot mode by accident:

I am trying to fix a 16GB Sprint Samsung Galaxy S3 that went through a rooting process and failed, leaving download mode and most other recovery options inaccessible.

I don't have a USB download mode jig or JTAG equipment, but I noticed it's recognized as "05c6:9008 Qualcomm, Inc. Gobi Wireless Modem (QDL mode)" when connected to a Linux computer.

From what I've read, not many people have gotten it out of this mode, however, I've found guides that imply the NAND/eMMC is writable in this mode with qdload.pl. None of the files I've seen want to be flashed with this tool - the generic, unsigned 8960_msimage.mbn/MPRG8960.hex files to get it into EMMC USB storage mode don't execute/stay intact after flashing, so I'm guessing they failed the signature check.

Is anyone willing to dump the partition table/bootloader from their working 16GB Sprint SGS3 for me?

dd if=/dev/block/mmcblk0 of=/sdcard/backup.bin bs=1048576 count=70

I unfortunately don't know what stages of the bootloader were corrupted, and I'm not sure what address to flash aboot to, if that's even possible with qdload.

Downloads:

Sprint SPH-L710: http://www.mediafire.com/download/231uhy6l80jx74n/debrick_sph_l710.img.xz (Thanks @CNexus!)

AT&T SGH-i747: http://d-h.st/iEy (Thanks @Android_Geeek!)

T-Mobile SGH-T999: http://www.mediafire.com/download/grgyera66w0rt25/debrick_SGH-T999.img (Thanks @Techlyfe!)