hi there guys and gals
i was just doing a routine weekly apt-get dist-upgrade
and i got the following errors

root@mahirhoca:/lost+found# apt-get install -f
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  linux-headers-3.13.0-79 linux-headers-3.13.0-79-generic
  linux-headers-3.13.0-85 linux-headers-3.13.0-85-generic
  linux-headers-3.13.0-86 linux-headers-3.13.0-86-generic
  linux-headers-3.13.0-87 linux-headers-3.13.0-87-generic
  linux-image-3.13.0-79-generic linux-image-3.13.0-85-generic
  linux-image-3.13.0-86-generic linux-image-3.13.0-87-generic
  linux-image-extra-3.13.0-79-generic linux-image-extra-3.13.0-85-generic
  linux-image-extra-3.13.0-86-generic linux-image-extra-3.13.0-87-generic
Use 'apt-get autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
4 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Setting up linux-image-3.13.0-91-generic (3.13.0-91.138) ...
Running depmod.
update-initramfs: deferring update (hook will be called later)
initrd.img(/boot/initrd.img-3.13.0-91-generic
) points to /boot/initrd.img-3.13.0-91-generic
 (/boot/initrd.img-3.13.0-91-generic) -- doing nothing at /var/lib/dpkg/info/linux-image-3.13.0-91-generic.postinst line 491.
sh: echo: I/O error
vmlinuz(/boot/vmlinuz-3.13.0-91-generic
) points to /boot/vmlinuz-3.13.0-91-generic
 (/boot/vmlinuz-3.13.0-91-generic) -- doing nothing at /var/lib/dpkg/info/linux-image-3.13.0-91-generic.postinst line 491.
Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 3.13.0-91-generic /boot/vmlinuz-3.13.0-91-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 3.13.0-91-generic /boot/vmlinuz-3.13.0-91-generic
update-initramfs: Generating /boot/initrd.img-3.13.0-91-generic
run-parts: executing /etc/kernel/postinst.d/update-notifier 3.13.0-91-generic /boot/vmlinuz-3.13.0-91-generic
sh: echo: I/O error
run-parts: /etc/kernel/postinst.d/update-notifier exited with return code 1
Failed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-3.13.0-91-generic.postinst line 1025.
dpkg: error processing package linux-image-3.13.0-91-generic (--configure):
 subprocess installed post-installation script returned error exit status 2
dpkg: dependency problems prevent configuration of linux-image-extra-3.13.0-91-generic:
 linux-image-extra-3.13.0-91-generic depends on linux-image-3.13.0-91-generic; however:
  Package linux-image-3.13.0-91-generic is not configured yet.

dpkg: error processing package linux-image-extra-3.13.0-91-generic (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of linux-image-generic:
 linux-image-generic depends on linux-image-3.13.0-91-generic; however:
  Package linux-image-3.13.0-91-generic is not configured yet.
 linux-image-generic depends on linux-image-extra-3.13.0-91-generic; however:
  Package linux-image-extra-3.13.0-91-generic is not configured yet.

dpkg: error processing package linux-image-generic (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of linux-generic:
 linux-generic depends on linux-image-generic (= 3.13.0.91.97); however:
  Package linux-image-generic is not cNo apport report written because the error message indicates its a followup error from a previous failure.
                                                                                                                                                No apport report written because the error message indicates its a followup error from a previous failure.
                                                                                        No apport report written because MaxReports is reached already
                                                                                                                                                      onfigured yet.

dpkg: error processing package linux-generic (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 linux-image-3.13.0-91-generic
 linux-image-extra-3.13.0-91-generic
 linux-image-generic
 linux-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@mahirhoca:/lost+found# 

what could be causing the i/o error?