The original 128 K and 512K "fat macs" came with 400K drives. These were single-sided and, because they could spin at various speeds (I believe there were five speeds), they were able to cram more data onto a 3.5" diskette than competitive 3.5" drives, which would hold about 270 K in machines offered by Heath/Zenith, for example. One of the major selling points was that these Apple 3.5" drives used a smaller, more compact floppy that, because it was encased in a rigid shell, could store data more tightly than could the competing 360 KB 5.25" floppy in the IBM PC/XT. Hence, the floppy held 400K on a single-sided disk whereas the IBM drive required both sides of the 5.25" diskette to store only 360 KB. Note that the IBM drives were capable of only one speed--they were far more primitive than the Apple drive.
After the 400K drive, Apple introduced a double-sided version, one of which I had for a little while. This held 800 kilobytes, and made a particularly endearing "mip-mmip-mip" noise as its head-actuators operated. (the older drive didn't make the noise, if I recall, but you could hear the motor whirring it's little five-note song as it changed speed during operation. The newer drives didn't sing. They just "mmmipped" along.
I remember loving that sound--kind of an "R2-D2" kind of thing, I guess....
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