Is the Photos To Disk third party app still available?
The website is available in the Wayback Machine, and version 2.1 can actually be downloaded through there, although the newer version 3.5 cannot be.
https://web.archive.org/web/20180609143433/http://photos-to-disk.com/downloadbuy.html
The bigger problem, though, is there's no way to buy a license key anymore, because the developer's FastSpring page is no longer available. Unfortunately, I don't think there's a way around this.
As a user of an older version of OS X (10.9), I run into this problem all the time while trying to find compatible software, and it's very frustrating! If you're a developer reading this, please consider making a public license key available somewhere before closing shop—future generations may thank you!