General :: How To Recursively Copy Directory Into Another One

I moved to Mac OS X recently and bumped into the "feature" of Mac where copying files from an external drive resets the file modification/update date/timestamp to the current date (which Windows does not), causing a disaster for my 10+ years of backup work files where date is important. So, before I learned how to avoid that (e.g. using the -p "preserve" flag in the "cp" copy command) I have in the meantime added to my new Mac hard drive many more files as well as updating existing old files.

I have a backup external hard drive with all my old data and proper modification dates. I have a Mac hard drive with reset modification file dates (a single or two particular days). The Mac hard drive has all the "true" and "current" file contents with files modified and added. I need to Copy all the original files from the external harddrive, preserving file metadata (really only modified date), but ONLY overriding the new internal Mac hard drive IF

The file contents (md5 or whatever) is the same or The file was updated after the day (which of course I can see on all files) on which the original disasterous cope was performed (implying the file is new or modified) Ensure the copy leaves all the new and modified files completely intact on the Mac internal hard drive. "No prompting/stopping of the copy of any kind (i.e., not verbose) is required but is o.k". "Recursive copy - obviously I would like to copy all* files folders and subfolders found in export".