The long-rumoured explanations went something along the lines of Amstrad getting them from a bankruptcy auction and/or salvaging them from a scrapheap after another computer manufacturer abandoned their project when it became clear that 3.5" had become the de facto standard.

Neither of those really made much sense - the choice would have been made about a year before 3.5" became unstoppable, and if there'd been a big computer manufacturer bankruptcy then why not sell off the whole computers rather than just the disk drives?

RP's explanation still hangs on Amstrad's reputation for being cheap'n'cheerful (reusing the ubiquitous 5.25" controllers & drivers definitely will have brought the cost down) but rings a lot more true than those other stories.

I was a 8512 owner so had the fancy-pants DSDD variant as my B: drive. The disks were great - not just more robust than 3.5", but the Amsoft-branded ones came with a jewel case with standardised inlay and labels. Even the soft slip cases you got with the Maxell ones were pretty useful!