If you have a smaller USB thumb drive you may discover that Windows tells you the drive is full when you try to save to it and yet when you check the properties of the drive you see that you have not used much space on the drive. This can occur when you have saved lots of small files to the drive
On a 1 gig USB thumb drive formatted using FAT (Drives can use FAT or NTFS systems to manage the files) there is a limit of 256 files and folders per folder. If you store all your items on the primary (root) level you will only be able to store 256 files on the drive.
The way to get around this is to create folders on the USB thumb drive just like is done on large hard drives. Then you are limited in each folder to 256 entries (files and folders) and you can store a lot more items on the drive. This does not change the amount of physical space that is available. This number can be smaller if you are using file names that are longer than eight characters as most of us do.
Basically what is happens is that the USB thumb drive that uses the FAT file system has a file that keeps all the names of the files and where they are. With file names longer than eight characters another special file is used. The file has a limit on the number of entries in it. On a 1 gig USB thumb drive this is 256 entries. It increases dramatically for larger drives and if the drive is bigger than 32 gig, we cannot use FAT but must use NTFS which is a different file management system that allows many more files and is much more efficient. However most non-Windows operating systems cannot use NTFS but for most people that is no problem as they only use Windows operating systems.
If you do run in to the problem of too many files on a USB thumb drive you can either go buy a larger dive (USB thumb drive prices have drastically dropped the last few years) or create a folder on the drive and move some of your files from the main (root) folder to the new subfolder and you will have the ability to store more files. If you are already at the limit, you may have to remove a file first to create the folder.