The Finder in OS X often gets confused as to what’s supposed to be in the “Open With” context menu, particularly if you mount and unmount other volumes that also contain an OS X system and applications.

This menu is handled by Launch Services which maintains a database of which applications are installed, and what file types they can open. Fortunately, it’s easy to rebuild this database.

You can either go into the Terminal and run the following command, or download a simple application to Rebuild the Launch Services Database.

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework
/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user

The linked application is simply an AppleScript saved as an application that runs the above command and alerts you when it’s done, or if there is an error.