USB sticks sometimes show less available memory than there actually is, even when you've removed everything from the stick. If this is the case, you will need to format your USB stick to resolve the issue. Here we show you how to do that on a Mac.
Plug the USB stick into your Mac and open the Applications folder, which can be found in the Finder, or by pressing the cmd key and typing applications.
In Applications, scroll down to the Utilities folder and open the Disk Utility application. Or search for the application using OS X's built-in search function.
Once you've opened the Disk Utility, you'll need to select your USB drive in the column on the left side of the window. Then select the Erase tab, at the top of the large column on the right side of the window.
On the Erase tab, ensure that the Format field displays MS-DOS (FAT). You can also name your USB stick in the box below.
The last thing to do is press the Erase button, and your USB stick will be formatted. Then the full capacity will be shown as available again.