gparted package in the Software Center (or in the Terminal/Konsole by running
sudo apt-get update followed by
sudo apt-get install gparted). This gives you the GParted Partition Editor.
Then run GParted. Find the MicroSD/USB drive in the list of devices in the drop-down menu at the upper-right corner of the screen. Make sure you have the right device. (You can look at its size to help gauge this.) It would be very bad if you accidentally formatted a different drive that had valuable data, or your Ubuntu system, on it!
With the correct device selected, run
Create Partition Table.... Use the default options (i.e., just click
Apply, don't change the option under
Advanced). Please note that, while other modifications in GParted are only performed when you apply your changes, creating a new partition table wipes out anything on the drive on which it is done, and is performed immediately when you click
Now create a FAT32 partition on the device (or a FAT16 partition if it is very small, like just a few megabytes, or if you know you need a FAT16 partition, for example, because you are following directions that call for a FAT16 partition). The new FAT32 partition should cover the whole drive (which is the default in GParted). You'll see the partition there, but that's just showing you what it will look like when you apply your changes.
Then apply your changes (by clicking the green check mark). Quit GParted.
You should be able to find the new partition in any Nautilus (i.e., file browser) window, under Devices on the left. Mount the partition by clicking on it under Devices in Nautilus.
Now you should be able to write to it with UNetbootin.