Recommended and mentioned products
Multi functional app to find duplicates, empty folders, similar images etc.
Download the latest FreeFileSync 10.4. FreeFileSync is a free open source data backup software that helps you synchronize files and folders on Windows, Linux and macOS.
Also, you can use FreeFileSync to mirror old NAS to new Synology and then use Starwinds deduplication analyzer tool to find out the data that could be deduplicated. https://freefilesync.org/ Https://www.starwindsoftware.com/starwind-deduplication-analyzer.
dupeGuru is a tool for finding duplicate files on your computer.
I would recommend using WinMerge or dupeGuru to compare two directories/locations and see the difference. Https://winmerge.org/source-code/ Https://dupeguru.voltaicideas.net/.