Raspbian hangs on upgrade

Problem

I tried to upgrade Raspbian using these commands:

$ sudo apt-get update
$ sudo apt-get upgrade

After the update, the upgrade downloaded and installed many packages, but hung with this last output:

Setting up xdg-utils (1.1.0~rc1+git20111210-6+deb7u3) ...
Setting up xserver-common (2:1.12.4-6+deb7u6) ...
Setting up xserver-xorg-core (2:1.12.4-6+deb7u6) ...
Setting up raspberrypi-bootloader (1.20150421-1) ...
Removing 'diversion of /boot/COPYING.linux to /usr/share/rpikernelhack/COPYING.linux by rpikernelhack'

Solution

On examining other processes running on this Raspberry Pi, I noticed Transmission. I tried to stop its daemon using:

$ sudo service transmission-daemon stop

It stopped, but its zombie process still hung around and could not be killed.

So, I tried to restart the Raspberry Pi using:

$ sudo shutdown -r now

It did not restart! Finally, I switched off the power and turned it back on.

As soon as I could SSH into it, I stopped Transmission, which worked this time.

I was able to finish the interrupted upgrade successfully, using this command:

$ sudo dpkg --configure -a

Tried with: Raspbian 7 (Wheezy) and Linux 3.12.35