Note that some of these devices require the USB drive to be formatted as FAT, not FAT32, which is common for larger drives. If you have a large drive, then you will probably have to partition it, and format the first partition as FAT.
Here's some instructions for how to do this under Windows 8, submitted by a user with this issue (WARNING - make sure you are partitioning the correct drive, otherwise you risk wiping out your hard drive!!):
1) In the run command field type CMD to start a dos window
2) At the C: prompt type -> diskpart
3) A secondary window will pop up requesting permission to run, click yes to continue
4) Once inside diskpart, type LIST DISK to see a list of the disk partitions on the computer
5) In the example instructions below, the two partitions listed are DISK 0 and DISK 1. We know that the thumb drive is 7538 MB we then begin typing out the instructions that proceed DISKPART> to proceed with removing the old drive partition, replacing it with a smaller value like 500mb or up to 2000mb. In the example below the disk partition is set to 500mb. (note, you can repeat these steps and restore the thumb drive back to it's original capacity by not issuing a size value when creating the primary partition. You can also replace FAT with FAT32 or NTFS as the format type)
6) Once the drive has been formatted correctly, copy the update1 and update2 (or the update.zip as appropriate) files to the root of the thumb drive and you are good to go.
DISKPART> list disk
Disk ### Status Size Free Dyn Gpt
-------- ---------- ------- ------- --- ---
Disk 0 Online 40 GB 0 B
Disk 1 Online 7538 MB 0 B
DISKPART> select disk 1
Disk 1 is now the selected disk.
DISKPART> list part
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Primary 500 MB 1024 KB
DiskPart succeeded in cleaning the disk.
DISKPART> create part primary size=500
DiskPart succeeded in creating the specified partition.
DiskPart marked the current partition as active.
DISKPART> format fs=fat quick
100 percent completed
DiskPart successfully formatted the volume.
DiskPart successfully assigned the drive letter or mount point.