This usually means you copied the .666XX files to a drive or the wrong folder. Reformat your USB to FAT32 using guiformat (for drives >32GB) or Windows built-in format tool.
The tool for PC is specifically designed to split PS3 game files larger than 4GB into smaller segments (typically with a .666xx extension) so they can fit on FAT32 formatted external drives. Core Feature & Setup split4g pc tool to split large 4gb ps3 fil exclusive