I recently had a problem with my OS X Lion MacBook Pro, and for one reason and another decided to address it by upgrading to Mountain Lion and installing from scratch, recovering any essentials from backups.

However, I ran into a real gotcha where my existing encrypted volume could not be erased, and so I could not proceed with the installation.

Luckily this blog post helped me resolve the issue by safely removing the encrypted partition, using a command version of diskutil in a terminal window, so I could reformat and then it could play nicely with the clean Mountain Lion installation Martyn Drake’s Blog