Picture Dropbox or Onedrive or some such, with their ability to keep files in the master storage online and only download them on demand, and later when I delete some files I have the option to either remove them locally ("dehydrate") but keep the master copy, or erase them from the storage, too.
So, with that in mind, I need that exact feature, but with an external drive (or several) instead of a network device. Copy files from the drive to local folders, have something keep track of changes and deletions, be able to either write changes to the drive again, or just free up local space but keep master files intact on the external drive.
The software should run on Windows. Price margin would depend on whether it could also sync to online clouds, etc. - but without a monthly fee for its own storage service.