There are adapters that let you use SD cards in place of floppy or IDE drive.

compactflash is nearly pin compatible with ATA, i had several laptops repurposed as home routers with CF cards on little adapters. I could probably dig one out and take a picture.

edit: shoot, i think i meant IDE, not ATA

Yep, my Amiga 1200 does exactly this. It’s very tempting to put it on the internet.

ATA and IDE is the same thing. Usually CF cards will just work on adapters.

Do they support hotplugging? afaik traditional floppies do and could be essential to workflows if you can avoid the costly reboot time.

There are adapters to use flash cards in real 3½" floppy drives.

I had a nice metal one made by Sony years ago. Flash card slides in one side, and there's a place for a button cell to power it. Then you just stick the whole thing into the floppy drive like a regular disk.

How many OS assumptions does it break to have a massively oversized capacity on a floppy drive?

Or is it just using 1.44mb of a multi-GB SD card?

The GoTEK (or similar) use several floppy images (each one 1.44 MB) and you can select manually which one to use.

I think it depends on the model, but should be able to select among up to 100 or 1000 such images.

Often, these present as multiple floppy sized chunks for exactly the reason you cite. I don't know about the Sony described, but the Gotek I have has a little front panel that let's you select a numerical floppy unit that presents as a: or whatever.

I'm assuming if it's a floppy emulator it just has floppy density, but you can emulate an IDE hard drive as well.

No, it is not a ATA/IDE (nor SATA) device, it is intended as a "direct replacement" for floppy drives (on embroidering machines, music instruments, CNC and similar), so it uses the 34 floppy connection (on PC-like hardware).

There is - besides Gotek - an alternative which is open source and (if one wants to) DIY, the HxC, a few different possibilities:

http://hxc2001.free.fr/floppy_drive_emulator/index.html

Oh, I may have been misunderstanding the parent comment but it sounded like they were saying it was an adapter that let you put flash memory devices of some sort into a regular floppy drive.

There are exactly such devices.

Neutron star density

GoTek being most popular, yes.