The 'disk full' problem cropped up when Microsoft first introduced long filenames. The root directory of a floppy disk has precisely 224 slots to hold directory entries. Before long filenames, that was enough for 224 files, and very few people ran into that limit. But each long filename takes from two to twenty directory entries, depending on its length. Some people have been getting 'disk full' errors even when a floppy disk was only half full. USB drives are typically formatted using the same FAT16 file system that's used for floppy disks, and they have the same problem. The root directory has a fixed size, and once it has been filled you can't add any more files.
Fortunately, there's a simple solution. Store your files in sub-folders. The number of directory entries in a sub-folder is variable, increasing automatically as needed. You can't create a sub-folder in a root directory that's already full, so move some files temporarily off the USB drive. Create a new folder on the USB drive and move all or most of the files into it. Once you've started doing this, you may choose to create other sub-folders and organize your files by category. The main thing is to stop dropping everything into the root directory. Now you'll be able to take advantage of the full capacity of your USB drive.