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It looks like you hit https://bugs.archlinux.org/task/23467.
Are you using [testing]? If yes, downgrade mkinitcpio to 0.6.8 (pacman -S core/mkinitcpio) and regenerate the initramfs image (mkinitcpio -p kernel26). To do that, you'll need to chroot into your installation using a live CD (instructions).
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