Did you know that OS X doesn’t by default merge folders?
This problem occurs when you try to move a folder into a location where there’s already another folder with the same name. The default behaviour in Mac OS X is to erase the existing folder, deleting all its files, rather than offering to merge them automatically.
On Windows and Linux, file managers try to handle this intelligently; they combine the files in the two folders into a single folder with the same name.
The Finder on Mac OS X isn’t so smart. By default, it just offers two options: Stop or Replace. Stop doesn’t do anything, while Replace replaces the old folder with the new folder. Replacing the old folder actually deletes all files in the old folder. The Finder doesn’t offer to merge the folders.
The Finder does actually have a folder-merging feature, but it doesn’t offer it straight away when you might want it. Instead, you need to use a shortcut key to access it.
You have to hold down the Option key as you drag-and-drop a folder into the same location. If you do this, you’ll see a dialog with a Merge button and you can click it to merge folders like you would on Windows or Linux.