When it comes to formatting bootable USBs, it is not recommended to use the Nautilus File Manager, since it does not provide features for formatting USBs in a specific way for bootable purposes. There are special software and tools which are needed for formatting bootable USBs. Specifically, to create a bootable USB, you may need a separate software tool to create the bootable environment on your USB drive and also to create the correct partitioning scheme. Nautilus only offers formatting options which don't have the detailed options that may be necessary to properly format a USB drive for boot purposes. To make sure that you properly format your bootable USB, you should use a dedicated software such as Rufus or Etcher, which provide the necessary tools to format it correctly.