PS3 SuperSlim HFW 4.89.1 not installing

I have a super-slim running 4.89 and I'm trying to install 4.89.1 HFW so I can get HEN running. However, every time I try and use System Update to get the HFW to install off the USB, it says that there's "No applicable update data was found".

The 8 GB USB stick is formatted FAT32 with the correct folder structure (PS3/UPDATE/) with the correct .PUP file. The PS3 is able to see the USB drive no problem as it shows up as a selectable option on both the Photo and Video tabs as a potential source device. I have tried plugging it into both of the front USB ports.

What am I doing wrong?