The best way to recover lost data from external hard drive is to follow some external hard drive data recovery guide, you may take a look at this one that helped me before how to recover files from external hard drive hope this help you get back lost files.

Dont worry, you can recover deleted pictures after accidentally format the system, here's a good format recovery guide that helped me last time how to recover deleted files after reformat You will need to connect the Nintendo DSi to computer, and then download this photo recovery software. hope this helps.