photorec

Edit: There's an even easier way to install PhotoRec now, assuming you have installed Homebrew, and you're comfortable in the command line:

brew install testdisk
sudo photorec

Then follow the prompts to start recovering files.

As a photographer who's taken and processed at least 200,000 photos in the past couple decades, you'd think I have a solid workflow that results in zero lost files... but you'd be wrong. 99% of the time, I follow the workflow:

1. Import photos from memory card.
2. Make sure backup of imported photos completes (so I have two local copies—I also have one copy back up to a cloud storage provider, so two local and one cloud backup).
3. Format the memory card.

A lot of photographers shoot with two memory cards, and have photos written to both—that way the 2nd card would be a double-failsafe. But for most jobs, I don't do that. And one of my digital cameras doesn't even have two memory card slots, so this isn't an option!

Anyways, more often than I'd like to admit, I do something dumb, like: