Sound Designer II Files. This format was a legacy format that's been around since the OS 7 days. It uses a thing called "resource forks" to store playback and metadata information. It was great in its time, but when Apple moved to OSX the ability to read this information was put into their Carbon framework. Carbon was an experiment to see what would prevail(Carbon or Cocoa), and to aid developers to port their OS9 apps to OSX. Well, Carbon is dead, the API's to access resource forks never got ported to Cocoa. Soundminer is now using the last Software Developer Kit(SDK) that Apple provided to make these calls. Its just a matter of time before we have to move to a newer SDK, and lose these functions all together. If you have a lot of SD2 files, before upgrading you should run Soundminer's mirror command to create new (broadcast wave) files with extensions. This will ensure your files will work fine going into the future(it'll also allow you to store the files on other file systems that Apple formatted drives!)