Double-sided disks, with notches on both sides, were available at a higher price, but in practice the magnetic coating on the reverse of nominally single-sided disks was usually of good enough quality to be used (both sides were coated in the same way to prevent warping, although only one side was certified for use). Early on, diskette manufacturers routinely warned that this technique would damage the read/write head of the drives or wear out the disk faster, and these warnings were frequently repeated in magazines of the day. In practice, however, this method was an inexpensive way to store twice as much data for no extra cost, and was widely used for commercially released floppies as well. Later, Apple IIs were able to use 3.5-inch disks with a total capacity of 800 kB and hard disks. DOS 3.3 did not support these drives natively; third-party software was required, and disks larger than about 400 kB had to be split up into multiple "virtual disk volumes." DOS 3.3 was succeeded by ProDOS, a 1983 descendant of the Apple ///'s SOS. It added support for subdirectories and volumes up to 32 MB in size. ProDOS became the Apple II DOS of choice; AppleWorks and other newer programs required it. The Apple II series of computers had an enormous impact on the technology industry and on everyday life. The Apple II was the first personal computer many people ever saw. Its price was within the reach of many middle-class families, and a partnership with MECC helped make the Apple II popular in schools. By the end of 1980 Apple had already sold over 100,000 Apple IIs. Its popularity bootstrapped the computer game and educational software markets and began the boom in the word processor and computer printer markets. The first microcomputer program for business was VisiCalc, the earliest spreadsheet, and it ran first on the Apple II. Many businesses bought Apple IIs just to run VisiCalc. Its success caused IBM to create the IBM PC, which many businesses purchased to run spreadsheet and word processing software, at first ported from Apple II versions. The Apple II's slots, allowing any peripheral card to take control of the bus and directly access memory, enabled an independent industry of card manufacturers who together created a flood of hardware products that let users build systems that were far more powerful and useful (at a lower cost) than any competing system, most of which were not nearly as expandable and were universally proprietary. The first peripheral card was a blank prototyping card intended for electronics enthusiasts who wanted to design their own peripherals for the Apple II.