First, be aware of file count limitations in the root. If your drive is formatted using FAT16 and you are copying data to the root of your drive, you may have reached the maximum number of file name and folders allowed in the root. You will not be able to copy any more files to the root. You can solve this by clearing some files out of the root and creating a subfolder. Move your files into the subfolder and you should no longer be restricted by the amount of files you are trying to store on your drive and will be only limited by the size of the drive. Subfolders do not have the same limits.
Second this may be due to a file size limitation in FAT32 - the most likely file system format on any USB Flash drive or Flash memory card larger than 4GB. Except for Microsoft's exFAT, there is no reliable way around this Microsoft limitation. exFAT has problems of its own that may be more significant than the 4GB file size limit of FAT32.
The following information describes a process that should be followed only by file system experts who understand and accept the consequences. NTFS is not specifically supported on Dane-Elec USB Flash Drives or Flash memory cards. NTFS was not designed for removable devices and could result in unexpected malfunctions including incompatibility with other computers and host, and complete loss of all data stored on the Flash device. Formatting and Reformatting will cause loss of all data currently on the Flash drive. Dane-Elec/ Gigastone is NOT responsible for the consequences of taking any of the following actions.
If you have Windows XP, in the Hardware Device Manager, you can set the "Write caching and Safe Removal" setting to "Optimized for Performance". This will then allow the Flash drive to be reformatted with NTFS, which will then allow storage of files larger than 4GB.
Remember: you should not attempt to use NTFS unless you are an expert on computer file systems.
be aware that NTFS was designed specifically for non-removable devices and so doesn't have any features that help protect data in the event of it being unplugged before being properly ejected from the host computer. When using NTFS, it's important to practice the "Safe Removal" procedures.
Next, it possible that the drive capacity only superficially appears to be enough to hold your data. A file takes up slightly more space on the drive than Windows Explorer describes.