Can't see files on usb stick - folders are there but no files inside

Hello

I plugged in my usb stick to download some pictures I have on there. The folders of the different albums are there and the info says the folders are of the right size (5gb for instance) but the files do not show so I can't copy them onto my Mac.
I tried it on a pc and it shows all the picture files and works fine.
I originally downloaded the pictures from my camera whilst abroad on a PC in a cafe and then transferred to the USB.
Anyone got any clues?

Thanks.....