I'm not sure about the snapdragon s3 but on the snaps s4 on the galaxy s3 if the processor fails to boot from the emmc (like trying to downgrade the bootloader on a triped e-fuse) it will then try to boot from the SD card (this has fixed a lot of hardbricked galaxy s3s and probably any other snap s4 phone)......another possibility is multibooting....I had a Sony xperia play and I could boot my main ROM along with 2 other Roms that were on my sdcard
Sent from my ATT Samsung Galaxy SIII using TapaTalk
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Current Device: Samsung Galaxy SIII (SGH-I747)
ROM: Stock Touchwiz Rom
Kernel: Stock 3.0.31-2024954
Previous Devices: Motorola Droid 1, Pantech Crossover, Motorola Droid X, LG Revolution, Sony Ericsson Xperia Play (r800x)