When you copied the data directory, you ideally wanted to make sure you preserved the file timestamps, since that's part of how Zotero determines whether files have changed and need to be uploaded. If the timestamps were all changed, Zotero would need to check every one against the file hash stored on the server. (And the same thing would happen if you used "Reset File Sync History" — that's not a troubleshooting step and you generally shouldn't use it unless we tell you to here.) But it shouldn't actually re-upload the files if they haven't actually changed. The sync button tooltip will show that it's processing the files, but unless you're actually watching upload data transfer, you may just be misinterpreting what you're seeing.