OS X Mountain Lion: If Time Machine remains in the “preparing” stage
After the initial backup, Time Machine backs up only files and folders that have changed since your last backup. To do so, Time Machine compares your current files and folders to those in the most recent backup. While this process occurs, Time Machine displays a “Preparing...” message.
The length of time Time Machine needs to prepare a backup depends on the amount of information on your computer, the size of the backup, and the speed of the connection between your computer and the backup disk.
The preparing stage can also take longer if:
The previous Time Machine backup was interrupted.
The computer was not shut down properly.
A disk was disconnected from your computer before being ejected.
A software update was recently installed.
You used your computer for several days without performing a Time Machine backup. This can happen if the backup disk has been disconnected.
Antivirus software can also cause Time Machine to take longer to prepare a backup. Make sure your Time Machine backup folder, located on the backup disk and named “Backups.backupdb,” is excluded from virus scanning.