In Windows, dragging a folder to a destination where a folder with the same name already exists results in the contents of the source folder being added to the contents of the pre-existing folder. If a file with the same name exists within both folders, then the version in the destination folder will be replaced, but otherwise, the contents of the pre-existing folder remain intact.
On Macs, although you will be warned first, the pre-existing folder as an entity will be replaced by the source folder. If you are not familiar with the traditional Mac behaviour, DATA WILL BE LOST. (If you immediately realize your mistake, OS X Finder can 'undo' this). According to the original poster (rozeel) in the thread, Finder is the oddball in this case, and Windows and Unix follow the merging behaviour.
This won't be news to a Unix user, but for a long time Mac-user but Unix dummy like myself, learning of the existence of a way to effectively merge different versions of a folder was a 'wow' moment. Read the rest of the hint to see a quick Terminal command to merge two folders together...
Given a source folder:
~/FolderX (containing files 1.txt, 2.txt, 3.txt)... and a destination folder:
~/EnclosingFolder/FolderX (containing files a.txt b.txt c.txt)Then the Terminal command:
cp -R ~/FolderX ~/EnclosingFolder
results in
~/EnclosingFolder/FolderX containing files 1.txt, 2.txt, 3.txt, a.txt, b.txt, and c.txt. The contents of the two folders are merged.
Some Notes: If you add an extra slash at the end of ~/FolderX/, then the contents of FolderX will be added to EnclosingFolder at the same level as FolderX, the result being more of an addition, not a merge. This works with CpMac -r as well. However, there are differences with cp when variations of the above syntax are used. If the two 'FolderX's both contain subfolders sharing common names, the merging occurs recursively within them as well, which is where this command becomes really useful.
[robg adds: This is a fundamental difference between UNIX, Mac, and Windows -- of the three, only the Mac uses the "replace" functionality, which can definitely catch a switcher by surprise. I know I'm always surprised the other direction on my Windows box at work -- 'hey, how come those files weren't erased by my copied folder?' Also a caution, though -- if you're not careful with cp -R, you can quickly find yourself doing more than you expected, and might lose something important in the process. Please back up regularly!]

