I needed to update the UEFI firmware on a new machine I had just bought, which involved copying the UEFI flash tool and the corresponding .CAP file to a USB drive. For the update to work, the drive needed to be formatted as FAT32 and set to bootable. I only had my MacBook around, and it turns out that the default drive formatter does not set the bootable flag.

When you open Finder and right-click the USB drive, you can erase the drive and format it as FAT. However, such drive fails to boot. Turns out that the correct way to do that is from the terminal:

$ diskutil list  # Find the disk identifier, say /dev/disk5.
$ diskutil partitionDisk /dev/disk5 MBR fat32 "MYDRIVE" 100%

Unlike the graphical utility, this command correctly sets the Master Boot Record, making the drive bootable.