Have you ever tried to copy some files from one folder to another folder with the same name but without overwriting the original folder's content? It's quite common for developer, like you need to add a series of files to your original folder. I encounter this situation in my Drupal development when I want to add a language to my site.

And I found that it is impossible to directly drag the new folders into the old one, this will replace all the old content. In my situation, i need to add extra translation folders in each modules.

And Mac OS come with a good tool for merging the folders into one. It's called FileMerge. It come along with the developer tool, if you can't find this in your machine, you probably need to install the developer tools from the Disc or download it from apple developer page.

Using FileMerge is easy,
1. Open the Software
2. Select the folders to be merged as left and right
3. Click Compare
4. Merge - by selecting the folders and select Merge -> Combines Files