Under Preferences in Premiere Pro 5.5.2 >Media>
Clean Media Cache Data Base.
What is this function and when should it be used ?
As declared in this help section, the 'Clean' button removes those Media Cache Database files from both Media Cache and Media Cache Files folders, for which associated source files are no longer available (e.g. for offline media). Having read that, one would probably expect that the 'Clean' button must delete all database indexes and auxiliary (cache) files associated with assets for projects one is not currently working on. However, that doesn't take place. So, the button should actually have been called 'Partially Clean Media Cache Database Randomly'.
Cleaning Media Cache Database manually, i.e. deleting all files from both Media Cache and Media Cache Files folders, not only frees up some disk space by removing junks that are no longer needed, but also helps fixing some Dynamic Link or Importer Generic Error issues.