This was made possible as the SuperDrive now utilitized the same MFM (Modified Frequency Modulation) encoding scheme used by the IBM PC, yet still retained backward compatibility with Apple's variable-speed zoned CAV scheme and Group Code Recording encoding format, so it could continue to read Macintosh MFS, HFS and Apple II ProDOS formats on 400/800 KB disks.
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Every Macintosh and PowerBook introduced from 1988-97 (with the exception of the PowerBook 100, PowerBook Duo series, and PowerBook 2400c, which offered a proprietary external floppy drive as an option), had a built-in SuperDrive floppy drive.
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This replaced the older 800 KB floppy drive that had been standard in the Macintosh up to then, but remained compatible in that it could continue to read and write both 800K (double-sided) and 400 KB (single-sided) floppy disks, as well as the then-new high-density floppies.
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The first Macintosh model to include a SuperDrive floppy drive was the Macintosh IIx (1988).
It replaced the 16 MHz Motorola 68020 CPU and 68881 FPU of the II with a 68030 CPU and 68882 FPU (running at the same clock speed); and the 800 KB floppy drive with the 1.44 MB SuperDrive (in fact, it was the first Mac to have one).