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It's an open question whether desktops would've kept using 5.25" until the end of the floppy era, but laptops meant something smaller was going to be introduced; that much was essentially predetermined. The contingent historical fact was the adoption of the particular 3.5" format we all remember, in preference to the many others that were contenders at the time.

I was reading https://en.wikipedia.org/wiki/Floppy_disk#%E2%80%8B3_1%E2%81%842-inch_floppy_disk just now and came across this: "In the early 1980s, a number of manufacturers introduced smaller floppy drives and media in various formats. A consortium of 21 companies eventually settled on a ​3 1⁄2-inch floppy disk (actually 90 mm wide) a.k.a. Micro diskette, Micro disk, or Micro floppy, similar to a Sony design but improved to support both single-sided and double-sided media, with formatted capacities generally of 360 KB and 720 KB respectively."

So the way Wikipedia puts it, sounds like the decision was basically made by committee. Twenty-one companies got together, carried out a sober evaluation of all the contenders based on technical merit, manufacturing cost, which influential members already had a large investment in what, etc, then issued a verdict and so it was done.

My understanding had been a bit different. As I understood it from e.g. https://www.folklore.org/StoryView.py?project=Macintosh&story=Hide_Under_This_Desk.txt the big breakthrough for the 90 mm format that ended up winning, was getting into the Macintosh, for which Apple helped Sony debug the drives (their own Twiggy drives developed for the Lisa, never having become reliable enough). I assumed this was the reason they started being used in PC compatible laptops, which settled the matter.

If that version of the history is correct, the outcome was determined not so much by a grand deliberate decision from all interested parties, as by a few particular events, decisions made by a handful of individuals who were trying to solve their own short-term problems; a historical accident, chaos at work in the technical sense of the word.

Which version is accurate?

So the way Wikipedia puts it, sounds like the decision was basically made by committee.

And that's what it was - and what made it succeed. A standardized disk format with a drive interface compatible to existing controllers.

As I understood it, the big breakthrough for the 90 mm format that ended up winning, was getting into the Macintosh [...]

Not really. For one, Apple used a Sony drive from before the standardization mentioned. While the mechanical and media part was the same, the drive did differ in its interface and operation, thus requiring dedicated controllers.

I assumed this was the reason they started being used in PC compatible laptops, which settled the matter.

More or less. There are eventually 3 major steps and some in-between development marking this process, with IBM's use of 3.5 inch drives in their PS/2 line as the final milestone.

• In 1980 Sony developed the 3.5 inch format. Only a few computers like the HP-160 or Sony's SBC-70 used that drive.




• In 1982 The 3.5 drive as we know it got defined by a joint committee. The attempt followed was to use Sony's mechanical and media design, but use an interface compatible (*1) to the existing Shugart standard for 8 and 5.25 inch drives. Only the connector was turned from PCB into a pinhead for size reduction. This had the advantage that all needed was a new cable to operate a 3.5 drive on existing 5.25/8 inch controllers.




• 1983 brought the first drives to this standard offering 360 KiB (single sided) or 720 KiB Double sided when operated at standard MFM controllers. Beside many small machines, a first batch of drives for MSX computers opened a door in the consumer market.




• Eventually the fist PC(-ish) Computer to use 3.5 inch drives was the Apricot PC in 1983




• 1983/84 was when Apple adopted a drive, based on the Sony design, but incompatible to the standard, for their Mac. The diversion was to increase storage and reliability at the same time. While it worked great, it's impact on the floppy marked could be ignored as Macs didn't gain much of a market share and the drive itself wasn't sold to other manufacturers.




• 1985 saw Atari and Commodore adapting standard compatible drives for their new 16-bit machines. Around the same time 3.5 inch also established itself as the standard format for MSX computers in Japan and Europe (*2). In combination these home machines created a huge user base lowering cost of drives and media at and below existing 5.25 inch drives.




• 1987 did IBM introduce their PS/2 line with 1440 KiB 3.5 inch drives (doubled as HD) as standard. Even thru PS/2 sales where, lets say, less than optimal, PC manufacturers rushed to embrace the 'new' format to show their advancement.

Shortly thereafter (1988 or 1989, depending on source) sales of 3.5 inch drives surpassed 5.25 sales ... and the rest is history.

• oh, and then there was ED (2880 KiB) in 1990, but that only caught on in Japan, despite IBM offering some PS/2 with ED drives.

*1 - Here hides the true secret, compatibility. It worked already well enabelign the move from 8 to 5.25 inch. At that time the 3.5 was deigned many new drive variations between 2 and 4 inch where developed. Most with their own, 'way' improved interface. None got a larger distribution - except such using a Shugart compatible interface. The 3 inch is a great example.

*2 - MSX2 made the 3.5 inch drive standard. 5.25 where still supported, but all manufacturer switched for 3.5 for their new machines.

This is covered in one of the major Mac history works, although I can't recall specifically which one.

When Jobs was putting together his supplier list the 3.5 had been standardized, as Raff notes, but you still had lots of companies pushing their own formats. Machines with all of these could be found on the market.

Jobs went to Japan to visit with the various manufacturers to see where they were, I don't recall anything suggesting he had made up his mind on the format (other than "no 5.25" anyway).

The account notes that in some cases he would be presented with mock ups, and in one case a block of material that was indicative of the size and shape of the proposed device. Apparently he savaged them in these situations, with the book joking that they went away to commit hari-kari after these meetings.

Only Sony had an actual production-quality drive ready to go at the production numbers he demanded. His numbers proved overly optimistic, but the rest is history.

It seems the history is similar to USB in many ways. USB was going to happen sooner or later, but the iMac certainly helped jump-start the process.