USB sticks can sometimes display less free memory available than you know to be true, even after you've deleted everything on it. When this is the case you need to format your USB to fix the problem. This is where we should you how to format a USB stick on a mac. See all storage tutorials.
How to format an USB stick on a Mac
Once you've put the USB stick into your Mac, you need to open the Applications folder, which can be located in the Finder - or by pressing ‘cmd’ and ‘spacebar’ keys together then typing "applications".
In Applications, scroll down to the Utilities folder and open the Disk Utility application. Alternatively, search for the application using OS X's built-in search function.
Once in the Disk Utility application, you need to select your USB drive in the column to the left of the window. Once you have done this you need to select the Erase tab, which is found at the top of the large column to the right of the window.
When in the Erase tab, you need to make sure the Format dropdown box is displaying “MS-DOS (FAT)”. You can also give your USB stick a personalized name in the field below.
Then next and last thing you need to do is press the erase button and you will have formatted your USB stick - it will then go back to displaying it’s full capacity as being available. (See also: Recover deleted files for free: recover lost data.)