Updated firmware so Time Machine could work after updating the Mac to Mountain Lion. Seems like a change in the AFP protocol prevented it from working.
Lost access to SSH because the firmware update deleted the authorized_keys file but left the sshd_config alone with PasswordAuthentication set to no.
Took apart MyBook World Edition by referring to the following 3 resources.
The next step was to connect the drive to an external hard drive case with a USB interface, which can then be plugged into a computer with Linux installed.
I used Ubuntu 12.04 and followed the steps at http://mybookworld.wikidot.com/forum/t-307537/mount-the-hdd-on-a-ubuntu-system to make the drive accessible through nautilus (the file manager).
Basically, open the terminal and run
sudo apt-get install mdadm
sudo mdadm --assemble --scan
The sshd_config file is in the etc folder in the 2.0GB partition now accessible in nautilus. All the partitions are mounted in /media
It is possible to open see the address bar and thus the full path to a folder by using the keyboard shortcut Ctrl+L in nautilus.
Administrator rights are needed to edit the file. Open the terminal window and navigate to the directory with the file.
cd /media/<random numbers>/etc
sudo vim sshd_config
Find the PasswordAuthentication line and change the value from no to yes. Save the file and it should be possible to login with a password again.
At the time this was written, I’m using the opportunity to transfer all the files back with the much faster USB interface, so this solution has yet to be tested.