That can be one of things:
- The memory stick is write-protected
- If using Linux the file system on the memory stick is corrupted. on linux running fsck will normally fix this, while on windows you would use chkdsk.
- The Memory stick is formatted in FAT16 and you have reached the limit of the number of files/folders in the root directory. The answer is to either format it as FAT32 or to move the books down a level as the FAT16 limit on the number of directory entries only applies to the root folder.