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When using the 2013 model Asus Nexus 7 flo to boot an image like one generated by mkbootimg --kernel <kernel> --ramdisk <ramdisk> --cmdline <cmdline> -o boot.img, fastboot is unable to boot the image.

After generating my image with mkbootimg --kernel origboot.img-kernel --ramdisk origboot.img-ramdisk.gz --cmdline "console=ttyHSL0,115200,n8 androidboot.hardware=flo user_debug=31 msm_rtb.filter=0x3F ehci-hcd.park=3" -o newboot.img and booting with fastboot boot newboot.img, the following results:

downloading 'boot.img'...
OKAY [  0.296s]
booting...

It is at this point that both the device and fastboot appear to hang. Turning off the device by holding in the power button yields the final two fastboot lines:

FAILED (status read failed (No such device))
finished. total time: 68.910s

At first I had assumed this error was my doing, seeing as I was mostly trying to get my own compiled kernels and hand written inits to boot. However, today (after a little more than a week of attempting to debug) I tried to use the mkbootimg binary compiled from this repo to rebuild the boot.img from the CyanogenMod 11 nightly release for my device and tried to boot it as above. When it failed in much the same manner, I used diff to get the difference between the official boot and my boot image. The difference was a few bytes in the header as follows:

origboot.img (0x13 - 0x23)

00 00 00 20 82 00 00 00 00 00 00 10 81 00 01 20 80

newboot.img (0x13 - 0x23)

00 00 80 30 11 00 00 00 00 00 00 F0 10 00 01 00 10

Besides from these two sequences, the rest of the respective files were identical to one another. As expected, correcting this seemingly defective sequence in the new file resulted into a normal boot into CM11 on my device