Makes sense. How would the phone get hard bricked if you can flash via Odin if you were to wipe the stock ROM slot ? The locked bootloader would still stay intact but the stock ROM would be gone. I am assuming the possibility of bricking the phone would be restoring the stock rom after it has been wiped while leaving the locked bootloader untouched. I was looking at safestrap for my friend and will pass along the information....too bad it is not like sprint where everything can be deleted haphazardly which is what I do when I flash new roms. I even format the internals so everything is completely empty when I put a new ROM.
When I was using SafeStrap I never ran into a hard brick myself, just soft brick that Odin would fix pretty easily. I'd say about 99% of the time that would be the case.
I'm not sure of the technical details as far as why messing with the stock slot can lead to issues. My guess , and i could be wrong, is maybe to do with the kernel. . SafeStrap Roms must be based of the same version if Touchwiz that the phone is on 4.2 4.3 4.4 etc and it must have the same kernel as well. Again I'm just guessing but when the phone boots up and runs all its checks, if there are any discrepancies i think that would lead you to a soft brick, again this is if you're flashing to the stock slot.
I think someone who is way smarter than me could explain the differences between what really is going on when soft bricks result from flashing to the stock slot vs a ROM slot. I'm just going off my experiences.
That said... Most people have no problems keeping the stock slot and running 2 or 3 ROM slots as long as they manage their internal memory well and send everything they can to external SD card.
I ran SafeStrap on my RAZR for a long time and for a little while on the s4 before I picked up an MDK build S4 that I could bypass the bootloader using Loki.
The RAZR had Kexec support or Bootstrapping, whatever you want to call it, which allowed for custom kernels and Aosp Roms. People are working on that for the s4 and hopefully that day comes because it opens up a ton of options as you can imagine.
Hope I helped a little. And like I said if you want more technical answers about SafeStrap and stock slot vs ROM slot just hop over to a SafeStrap thread I'm sure someone will know
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