What is wrong with my flashdrive?

Hi Y’all. I have a 4GB Kingston flashdrive for a usb. When I plug it into any computer, and view its files/open the drive contents, there is nothing to be seen!

However, it says that nearly 2.5 gigs of it is full.

When I head to a Walgreens and plug it into the manual photo print machine’s usb, hundreds of pics show up. How can I access these files? Are they hidden? What can I do to clear space on my flashdrive?