Corel Systems Corp of Chicago reckons that its Corel Worm, storing 400Mb per side of a write-once optical disk, has the highest capacity of any drive offered for MS-DOS computers on the market: it fits a full size 5.25 housing, is available in internal and external versions, and costs $4,000 for the first, $4,400 for the second, and the optical disks cost $225 apiece.