Once you have gnome-disks
running, select the proper drive. From the top-right gear, select "Format..."
Unless you know why you would want something different, you should select "MBR/DOS" (partition table type)
If confident you have the right disk, go ahead.
and you should end up with a disk with an MBR partition table and no partitions
Next, add the partitions you need. Select the free space and click the "+" beneath the map of the partitions
I am setting this one up for Hefe Hook and external data, with a partition for sd-ext and a spare, so I only allocate 54 GB to the FAT partition. If you only need a FAT partition, you can just let it use the entire disk.
After created, you should see it in the partition map. If you used the whole disk, you won't see the "Free Space" section.
If you're adding more partitions, for sd-ext, or other reasons, select the "Free Space" section, then click the "+" again.
Here I'm adding an ext4 partition for "app-to-SD" scripts and other legacy purposes.
Here is the four-partition disk that I am using today.
If you have problems like it not allowing you to convert from GUID to MBR partitioning, the following will wipe the beginning of the disk and generally let you start with partitioning. Insert the right drive letter in place of the # below
Make sure that you select the right drive, as you can wipe your system or any other disk you have with this!
sudo dd if=/dev/zero of=/dev/sd# bs=4k count=1k