[7" Kindle Fire HD Q&A, Help & Troubleshooting] peterpion:
Well I fixed it, thought I should post back here as its an interesting fault.

Turned out 2 of the pullup resistors were open circuit, obviously my fat fingers and bad soldering! I found it because looking through the datasheet for the emmc chip last night, I noticed that the chip needs pullup resistors on several of the pins (last page of KLM8G2FE3B datasheet which you can find online). Thinking back to the mainboard in the KF2, I remembered the grid of resistors and thought maybe they were the pull up ones. Reason I was thinking about this at all was I wondering if the chip can be ripped out and replaced with a very large SD card, but thats for another day.

Anyway I thought I might buzz out the data lines and CMD as these are the ones with the pullup resistors, to check my soldering. IE I should see the same resistance on each (with respect to +ve). Well 3 of them were 10k, which seems about OK although the datasheet recommends 50k, but this seems in the right range. But 2 were 450k ish. Far too high. If the resistors were blown then this is the kind of resistance I expect to see, back through the memory chip rather than through the pull up resistors to +ve. The bad lines were data1 and CMD btw. Really hard to solder, and I ended up having to solder to the resistor on both, with the result that the resistors blew.

The reason the emmc chip seemed ok when plugged into the computer is probably because the SD card reader provides these pull up resistors in it, making the ones on the board redundant. When the SD is unplugged, there is no pullup on those lines, and the chip is hence not readable - although I am still confused as to how it booted at all, maybe initial boot is only at a reduced data rate which didn't need the pullups, I don't know.

Anyway I put a couple of 10k resistors I had laying around between the still soldered on SD adapter and +ve, so effectively replacing the on board pull up resistors with the external ones. I was then able to install CM and get the tablet running. Since there was not room in the tablet for these large resistors etc, I spent several hours today trying to solder on tiny wires, and solder a couple of smt 10k resistors in place of the test ones. I succeeded eventually, with the wires coming from the rear of the main pcb where most of the wires connect in the 'rescue kf2 with soldering to chip' thread, and then the wires go round the pcb edge to the other side (that you see when you take the back off the tablet). Theres more room this side, and I soldered the new smt resistors on to the wires, and eventually got it working again.

I didnt replace the smt resistors where the old ones went because I simply can't solder this small. It was hard enough connecting those 2 wires to the 2 solder pads. Took hours. Not worth it considering I only paid 13 quid for this tablet but satisfying to fix.

I put this here in case anyone ever finds it useful - perhaps for other things. I still wonder very much could the emmc chip be ripped out - perhaps a large soldering iron held on the back of it for a couple of mins would allow it to float off - and could a micro SD be connected to the 8 solder points used for the rescue project? Looking at the emmc datasheet it seems exactly like an SD card. It can be read and written like an SD by an external computer as we can see from the success of the recovery project. I have made microcontroller circuits (atmega etc) which read and write SD cards and its not rocket science. Part of me wishes this KF2 had been unrecoverable so I could try it. Its something I might try if I pick up another kf2 sometime. But for now enjoying tinkering with this one. I intend to study the datasheet a bit more, see if its completely compatible with the SD protocol, and if so try it. Might post another thread sometime asking some of the more experienced guys here if they have any idea if it will work.