LMP Disk Doubler is a converter kit for MacBook and MacBook Pro Unibody:
It allows you to install a second SATA hard drive or SSD, instead of the optical drive. Almost all MacBook and MacBook Pro Unibody are supported. Please check the technical specification for further details.

A possible configuration is a super fast SSD (up to 500 MB/s transfer rate) in combination with a standard hard disk. Or expand the capacity by two 2 TB hard drive for maximum 4 TB and use a hard disk as a Time Machine backup.
There is even the possibilty to set up a RAID volume.

The removed DVD drive can be mounted in an inexpensive USB enclosure
(P/N 9413), so that it becomes externally available.