Just to check - you have the update unzipped, with the folder "UD_B5A1" copied to the USB root directory? (I.e. when you open the drive in Windows Explorer, it just shows a folder with that name; if you open the folder it should have several files with names like UPDATE0.UPD.)
Did mine on Friday, no problem. Used the same USB drive from the previous update, and just verified it's formatted as FAT32.
Edit: One other thing I found is that FAT32 can only be used for smaller drives, 32GB or under. So if your USB is 64GB or more maybe try a smaller one and see if you can format to FAT32.