[lproven]

Ha! 3" or Microdrive? They were different things.

The ZX Microdrive was the Sinclair stringy-floppy, released 1983, giving approx. 85-95 kB per cartridge: https://en.wikipedia.org/wiki/ZX_Microdrive

But 3" was a true floppy disk, used in e.g. the Amstrad Spectrum +3, released 1987, where it gave 180 kB per side: https://www.old-computers.com/museum/computer.asp?c=222

I love that people are still making enhancements for these ancient machines. For example, Amstrad promised an add-on disk interface for the cassette-based Spectrum +2, but never shipped one. Now, the nonexistent hardware has been cloned and you can buy a new one!

https://www.8bits4ever.net/product-page/sdi-1

There's an SD-card based replacement mechanism for the original Microdrive:

https://vdrivezx.com/vdrivezx/

[pjmlp]

I meant the Amstrad Spectrum +3 one.

I was envy of a friend that had one due to CP/M support.

[lproven]

OK. The Spectrum +3 had true floppy disk drives -- Hitachi 3" units. They are not Microdrives, are not compatible with Microdrives, and Microdrives cannot be attached to a +3.

Sinclair's system was much older (4 years, a long time then), and had its own external controller, the Interface 1. Microdrives were like tiny 8-track cassette tapes: an endless loop of tape on a single tiny reel, feeding out from the centre and wound back on the outside via a twist. They cannot be rewound or run backwards, only fast-forwarded, so access was slow.

So, no: not even similar. Different size, technology, OS extensions, interface, capacity, speed... different everything.

I had a Microdrive setup in the early 1980s. Like much Sinclair technology, it was radically cheaper than the competition. 90 kB of storage isn't much but it was twice the total RAM capacity of the host computer, and was 10x or more faster than cassette tapes. A microdrive cartridge could hold dozens of BASIC programs or machine-code snippets.

The Sinclair QL semi-16-bit computer also used Microdrives, with 2 built in, but with a different, incompatible format that got slightly more data storage (maybe 100 kB up to 105-110 kB if you were very lucky).

There were multiple officially-licensed derivatives of the QL, mostly running different incompatible OSes, and they mostly used Microdrives too: the Merlin Tonto, ICL One-Per-Desk, Telecom Australia ComputerPhone and more.

3rd party clones such as the CST Thor replaced the microdrives with floppy disk drives -- more expensive, but much faster and much more reliable.