The Macintosh External Disk Drive is the original model in a series of external -inch floppy disk drives manufactured and sold by Apple Computer exclusively for the Macintosh series of computers introduced in January 1984. Later, Apple would unify their external drives to work cross-platform between the Macintosh and Apple II product lines, dropping the name "Macintosh" from the drives. Though Apple had been producing external floppy disk drives prior to 1984, they were exclusively developed for the Apple II, III and Lisa computers using the industry standard -inch flexible disk format. The Macintosh external drives were the first to widely introduce Sony's new -inch rigid disk standard commercially and throughout their product line. Apple produced only one external -inch drive exclusively for use with the Apple II series called the Apple UniDisk 3.5.
400K
The original Macintosh External Disk Drive (M0130) was introduced with the Macintosh on January 24, 1984. However, it did not actually ship until May 4, 1984, sixty days after Apple had promised it to dealers. Bill Fernandez was the project manager who oversaw the design and production of the drive. The drive case was designed to match the Macintosh and included the same 400-kilobyte drive (a Sony-made -inch single-sided mechanism) installed inside the Macintosh. Although very similar to the 400-kilobyte drive which newly replaced Apple's ill-fated Twiggy drive in the Lisa, there were subtle differences relating mainly to the eject mechanism. However, confusingly all of these drives were labelled identically. The Macintosh could only support one external drive, limiting the number of floppy disks mounted at once to two, but both Apple and third party manufacturers developed external hard drives that connected to the Mac's floppy disk port, which had pass-through ports to accommodate daisy-chaining the external disk drive. Apple's Hard Disk 20 could accommodate an additional daisy-chained hard drive as well as an external floppy disk.
3.5-inch single-sided floppies had been used on several microcomputers and synthesizers in the early 1980s, including the Hewlett Packard 150 and various MSX computers. The standard on all of these was MFM with 80 tracks and 9 sectors per track, giving 360 KB per disk. However, Apple's custom interface uses Group Coded Recording (GCR) and a unique format which puts fewer sectors on the smaller inner tracks and more sectors on the wider outer tracks of the disk. The disk speeds up when accessing the inner tracks and slows down when accessing the outer ones. This is called the "Zoned CAV" system; there are five zones of 16 tracks each. The innermost zone has 8 sectors per track, the next zone 9 sectors per track, and so on; the outermost zone has 12 sectors per track. This allows more space per disk (400 KB) and also improves reliability by reducing the number of sectors on the inner tracks which had less physical media to allocate to each sector.
The external 400-kilobyte Macintosh drive will work on any Macintosh that does not have a high density SuperDrive controller (due to electrical changes with the interface), but the disks in practice only support the MFS file system. Although a 400-kilobyte disk may be formatted with HFS, it cannot be booted from, nor is it readable in a Mac 128 or 512.
Copy protection schemes were not as elaborate or widespread on Macintosh software as they were on Apple II software for several reasons. First, the Mac drives did not afford the same degree of low-level control. Also Apple did not publish source listings for the Mac OS ROMs as they did with the Apple II. Finally, the Mac OS routines were considerably more complex and disk access had to be synchronized with the mouse and keyboard.