There are two external hard drives
Drive A has somewhat less than 600 GB consisting of almost 115,000 files in more than 2000 folders.
Most data is audio or video, predominantly audio.
Audio is mainly music or books.
Each music album or other collection is in its own folder. Each book is in its own folder. Call these folders data folders.
There is a tree structure of three major categories, each with sub categories going three or four deep before there are any data folders. In various categories, some folders may be data folders, others may contain sub category folders each with its data folders one or two steps deeper.
Drive B has the same basic structure but about 100GB less data. This can mean some sub categories are missing entirely.
I need to add everything on A not on B to B.
Nothing goes from B to A.
Anything already on B but not on A is left alone.
Nothing within a B data folder is deleted or changed.
Nothing is added to an existing B data folder.
Ideally, if an A data folder has different content than a B data folder of the same name, its entire content should be added to B with a modified data folder name. If this is too difficult, the data content should not be compared; the analysis is thus purely at the folder level: If the folder doesn't exist, copy it and its content, if the folder exists, assume its content already matches.
I do not want to need to spend dozens of hours trying to figure out what to copied, software should do this.
Is there anything available that can accomplish this? Does Windows have such a function?
I've been doing exactly that (but for my photograph collection) with Syncback (http://www.2brightsparks.com/syncback/sbpro.html), but, as you may have already found out, there are many similar software which can meet your requirements.
Thanks for the link, this was a helpful tool for me as well.