[Nexus 7 Q&A, Help & Troubleshooting] bftb0:
Well, because you have a pure stock tablet and the problem occurred spontaneously and under innocuous operating conditions, the most likely explanation is some type of hardware failure affecting the eMMC NAND chip.

Even if that is the case ... AND you successfully are able to get to fastboot mode ... it still might not be repairable. It could be a modest failure (e.g. something affecting the cache partition only, which could knock offline both the normal boot as well as the recovery boot), or it could be an enormous fraction of NAND.

eMMC NAND is supposed to be able to degrade gracefully, so there is a small chance that erasure and formatting of partitions will release blocks that have failed. But it could also be that the failure is extremely severe and no recovery is possible even if the bootloader is still intact.

I imagine you've already tried it, but since it will be your only possible means of rescue I would try the bootloader interrupt trick (Vol-Down approximately when the b&w Google logo appears - hold it down for 3-4 seconds)

Since the tablet gets stuck in a hardware reset loop, you should probably unplug the battery after every failed attempt, start it up normally (power button only), and then press and hold Vol-Down as soon as you see the Google b&w logo. Perhaps with the use of a stopwatch, you could experiment with slightly different timings, possibly even try the button press a second or two before you expect the logo to appear. I am not aware of any other way to get to fastboot (without having a booted kernel as in the recovery or normal os); and it may also be that the only time that the Vol-Down button has any effect is under the "default" (empty) BBC contents. If that is the case you are probably SOL.

The objective is to try and get the tablet into fastboot mode by any means possible.

If it were happening to me, and I was able to get that to happen, the next thing I would do would be to erase the entire tablet (except the bootloader) using fastboot, and then follow that up with an installation of a stock recovery (and formatting of cache, system, and userdata in that order), and then follow that up with a "factory reset" using the stock recovery.

If you can get just the recovery and (bootloader) fastboot mode working again, there is an outside chance that the tablet could be put back into service.

A little bit of explanation is in order here for me to explain why I am suggesting this. Sometime after I got my tablet, I spent a bunch of time dumping *all* the device partitions (things like MISC, USP, PERS, etc in addition to the "normal" Android partitions) before/after certain fastboot operations. What I noticed - by accident - was that I was seeing data that belonged in a certain partition showing up in other partitions. And always in block-sized offsets.

I think that what I was observing was an artifact of the eMMC wear leveling / block remapping that is part of the device's FTL (Flash Transition Layer). Literally, blocks were being shuffled around - even across partition boundaries - when they were in an unused/erased state.

In addition, if you look at the Google instructions for flashing a factory image, you will note that sequence is

erase p1

erase p2

...

erase pN

(followed by)

format p1

flash p2

format p3

...

That is, **as much as possible is erased before anything is put back into service**

It is my presumption that this happens to give the wear leveling and bad block remapping process of the eMMC chip maximum flexibility (as most everything in the device is marked as not in service it should be easy to remap based on block write counts as individual partitions are put back in service via via flash and format operations). Can't prove that though.

It looks like you did your reading about APX mode - there is a risky and fairly complicated procedure available to rooters that allows them to capture some custom (unique per tablet) file blobs that allow re-installation of a bootloader from APX mode, but is a procedure that has to be performed ahead of time. (And because the file blobs are unique per tablet - encrypted & signed with a unique hardware identifier, you can't get someone else to generate them for you) So APX mode is of no value to you afaik. It's barely of value to rooters as well. (I will say that I haven't kept up over the last year, so there is always the chance that someone discovered something new wrt the Tegra3 and APX mode that I don't know about - but that's the way it was a year ago or so)

Well, good luck with your tablet - I suspect you are going to need a bit of good fortune.