Either,
you have a very old PC that doesn't support hard drives above 2.1GB, even though the bios will show the real size.
or,
You only have a 2.1GB partition on your drive, and need to create a larger partition to see the additional capacity.
If you posted more info, such as your mainboard make/model, then we wouldn't need to guess. You need to format with FAT32 or NTFS to get larger partition size. Standard FAT (i.e. FAT16) only supports up to 2.1GB.