Source: Apple – Support – Discussions – Can’t reset to dock defaults? What worked for me: delete the files com.apple.dock.plist and com.apple.dock.db from homedirectory/Library/Preferences and restart Dock. To restart dock either quit it in the activity monitor or enter the following command in terminal: killall Dock The dock will restart and it will be rebuilt [...]
From a 20-in. aluminum iMac to a 2.4 GHz, 15-in. MacBook Pro (2008) and 2.93 GHz iMac (2009)