If you’ve never heard of these … the Internet is a truly big place. Both of these are by Christophe Grenier:
https://www.cgsecurity.org/wiki/
https://en.wikipedia.org/wiki/PhotoRec
YMMV - but - I recently used Photorec to recover >50,000 of images (including jpg, png and RAW) from a Mac RAID drive that the Mac could no longer read - not even in a USB caddy.
Now obviously, this won’t blow anyone’s hair back on an SBC but it could be pretty useful in an emergency to pull valuable images from (say) a memory card attached to a USB card reader. It’s more than just photo recovery and can even scan for things as diverse as Skyrim save games and database files… pretty neat stuff. 
I’ve not had any chance to run Testdisk but it’s highly respected and been developed for a number of years. Like PhotoRec it reads all manner of odd formats. God I love DietPi and I don’t care who knows it.