"Inaccessible boot device" error, but BCD, partition and drivers seem correct. What else could cause it?

Question

My Win8.1 x64 EFI laptop refused to reboot after a recent update, the error is INACCESSIBLE BIOT DEVICE but I can't figure how to debug this. So far this is what I've done:

-Used DISKPART to assign letters to all partitions but the 299MB non-data base partition (which is RAW formatted), and checked all partitions expected to be there, were listed. - chkdsk /f/v/offlinescanandfix to check each partition. Rum twice for each volume. - bcdedit enum all and checked that all devices, osdevices, ramdisk device references and paths exist and are correct. (Every device/osdevice lists a partition=X: not a HarddriveVolume, showing that it references a valid volume/partition). Then checking all options/entries for the bootmgr and win8.1 entry against a working boot on my Win8.1 desktop to ensure they're valid. - dism /image:X:\ /cleanup-image /restorehealth (where X is the system drive's temporary letter), run twice.

Booting into recovery (on its own partition) works fine, as does booting to set startup options on the Win8.1 item. But booting the Win8.1 entry normally, or into alternative modes (safe mode, safe mode+command prompt, disable early launch malware drivers), the Win8.1 startup "spinner" displays for a while, and then I get a BSOD with the error "INACCESSIBLE BOOT DEVICE".

An attempt to check drivers failed - the boot log file isn't being saved (dir /s nt*.txt on each volume on the disk doesn't list anything). The disk itself is an M.2 Liteon SSD and the chipset is Intel Haswell mobile (dont know the exact chipset) with the disk in AHCI mode - the firmware doesnt have any other options so it's never been used with any but the default driver.

It was working fine right up to when I restarted after the recent update.

Every case of inaccessible boot device I know of, is either hardware, BCD or incorrect driver error. But as best I can tell, these are all okay. What else could it be, and what can I do to trace the error? I'm comfortable running more advanced approaches, I could reinstall but I'd like to avoid that if possible.