file_save() & hook_file_insert(): Intercept before file is saved to disk?

Hi

If I read the File API correctly, when a file_save() operation is requested, then a file is persisted to disk and then it file_save()'s associated hook_file_insert() & hook_file_update() implementations are called. Is this correct?

If this is so, then can is possible to prevent file_save() operation writing to disk?

Thanks

Jeff in Seattle