upgrade error 16.04 / hang on sync after cpio build of initramfs

When upgrading (apt-get upgrade) on 16.04 (ubuntu-mate) I get a hang (left from 9:50am til 16:00 or thereabouts)

I tracked down as far as the call to /etc/kernel/postinst.d/initramfs-tools and added a -v option, the last relevant lines on the terminal when re-running `apt-get upgrade` are as follows:

...
Adding binary /bin/date
Adding binary /sbin/hwclock
Adding binary /sbin/dumpe2fs
Calling hook dmsetup
Calling hook reiserfsprogs
Building cpio /boot/initrd.img-4.4.0-24-generic.new initramfs
^CFailed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-4.4.0-24-generic.postinst line 1052.
dpkg: error processing package linux-image-4.4.0-24-generic (--configure):
 subprocess installed post-installation script returned error exit status 2
dpkg: dependency problems prevent configuration of linux-image-extra-4.4.0-24-generic:
 linux-image-extra-4.4.0-24-generic depends on linux-image-4.4.0-24-generic; however:
  Package linux-image-4.4.0-24-generic is not configured yet.

Checking with `ps fax` before the interrupt, I get this tree - indicating a hang on `sync` task

sudo dpkg --configure -a
 \_ dpkg --configure -a
     \_ /usr/bin/perl /var/lib/dpkg/info/linux-image-4.4.0-24-generic.postinst configure
         \_ run-parts --verbose --exit-on-error --arg=4.4.0-24-generic --arg=/boot/vmlinuz-4.4.0-24-generic /etc/kernel/postinst.d
             \_ /bin/sh -e /etc/kernel/postinst.d/initramfs-tools 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
                 \_ /bin/sh /usr/sbin/update-initramfs -v -c -t -k 4.4.0-24-generic -b /boot
                     \_ sync

I tried moving the resulting initramfs img file in /boot so a new one would be created - a new one is indeed created, hang still occurs on `sync`