A two-spindle system, also called a twin-spindle system, is a computer design with two internal storage drives. These are usually the hard drive and an optical drive. A third drive, usually a floppy disk drive, is not included. The two-spindle design is popular in ultra-thin notebook computers because it minimizes bulk and mass, and also keeps the cost down.
In a two-spindle system, installing an additional drive (such as a floppy drive or a second hard drive) requires that one of the permanent drives be swapped out. Usually this is the optical drive (CD-ROM, CD-R, CD-RW, or DVD), which is installed in a removable (modular) drive bay. Two-spindle computers are more convenient than one-spindle (or single-spindle) portable computers, in which only the hard drive is internal.
Full-size notebooks, also called desktop replacements, often employ a three-spindle design with an internal hard drive, optical drive, and floppy drive. These computers are heavier and bulkier, as well as more expensive, than the ultra-thin notebooks, but some users prefer them because the three-spindle design minimizes the number of external accessories.