Just in case somebody else had problems, I had 2 out of 3 Pogoplug Mobile that failed to boot after latest 4.3.0-1 kernel update.
Both of them could not chroot to ROOTFS and could not fsck it with a message to the effect of PARTUUID-0000... not found.
I have looked at the MMC cards I use for boot and two of them somehow were showing LABEL and UUID but not PARTUUID. I could not find how to assign PARTUUID to a partition , so I have backed up contents of the boot cards and re-formatted them per installation instructions . PARTUUID has shown up and after restore both machines booted OK.
My guess is previous version of the initramfs image used UUID to pass to kernel , but the newer one requires actual PARTUUID.
Correct me if I am wrong.