Why does the Playstation not recognize the USB?

I'm the proud owner of a Ps3 Slim and a Ps3 Fat. My problem is that my ps3 slim does not recognize the usb stick but the ps3 fat does. I can't play on the ps3 slim because when I switch it on the following comes up:

And when the playstation checks the USB stick, it checks all the time and nothing comes up with "4.86 was found and will now be installed. The USB stick is Fat32 formatted I can't explain why the ps3 slim doesn't recognize it do you have any tips?

Is the update file in the "PS3 / UPDATE" directory on the stick?

Yes