But my computer recognizes only 8.4 gigabytes or so. This is known as the 8.4 Gigabyte Barrier. It isn't very common anymore. Older BIOS do not recognize larger hard drives. You have a few options.
None are cheap.
You can get a BIOS upgrade, which is a piece of hardware you install into your computer. You'll need to find out if it will recognize drives larger than 8.4 gigabytes, though, because some may not. You have to exercise extreme caution when performing a BIOS flash, however. If done incorrectly, it could cause serious computer problems. Furthermore, flashing the BIOS may not break the 8.4 Gigabyte Barrier.