When trying to copy a folder onto another of the the same name in OSX you will only get prompted to replace the items and not merge them. This is a real pain in the ass if you’re trying to merge folders and if the folder has several other nested folders.
I just came across this exact scenario trying to update the Flex SDK and copy in the Datavisualisation components (as instructed here) because it just replaced the SDK!
Anyway to resolve this copying from terminal can behave like you would expect:
cp -R -n /SoureFolder/* /TargetFolder/
so in the case of the datavis components:
cp -R -n /Users/[username]/Downloads/datavisualization_sdk3.5/*
/Applications/Adobe\ Flex\ Builder\ 3\ Plug-in/sdks/3.5.0/