Question: I'm aware there are floppy emulators that are installed to 3.5″ bays where disk images are stored on a flash drive. But what I need now is the opposite: essentially a USB dongle that emulates the floppy drive hardware with a disk inside. Something that older OS installers would recognize.

The need came up when I tried to install Windows XP on a laptop. The only way to load the AHCI drivers is via a floppy drive. I have a USB floppy drive, but no disks at the moment :( and obviously there is no 3.5″ bay to install the typical emulator in a laptop.

I know I can always just buy some floppies, and in fact I have some on the way, but given the reliability of floppies over time, I still feel that what I'm describing would often come in handy.

Does anyone make/sell these?

Answer: 
I don't think anyone sells something like that in one piece.

There are, however, components on the market that should allow you to build that from scratch:

1. A GoTek or HxC that behaves like a "real" floppy
2. An *old* Floppy-to-USB adapter that was used to connect "real" floppies over USB. I don't think they're still made, so you would need to source one from eBay. Newer USB floppy drives no longer have this as I have learned from answers to [this](https://retrocomputing.stackexchange.com/questions/5428/is-there-anything-useful-for-a-retro-in-these-cheap-chinese-floppy-drives) question.
3. Some sort of external power supply, as the GoTek/HxC will not be willing to live from the USB power supply.

Putting it all together would end you up with something that behaves like a real floppy, connected over USB.

This is, however, never going to be a full replacement for a "real" floppy disk drive. Old computer's abilities to for example boot from USB floppies have always been very limited (even if they could always boot very well from standard floppy drives). Once you find one that does this, it will most probably also boot from a standard USB flash stick.

You'd probably be much better off by buying a bunch of HD disks and storing them well.

Another, entirely different, but possibly long-term method to make Windows XP think it has a floppy drive would be a *[virtual floppy driver](http://vfd.sourceforge.net)*. This just emulates a floppy based on an image stored on hard disk and could be a solution for many problems. You'd obviously need to have a drive and disk first in order to pull the images from "real" disks.
To boil down what others are saying:

A USB device itself is incapable of *making itself appear* as a traditional floppy drive, because USB devices don't have that kind of access.

However, if you're lucky, your BIOS will have support for USB floppy drives and *it* can apply the same virtualization trick that it uses to present USB keyboards and mice as PS/2 keyboards and mice for compatibility with old OSes.

If your BIOS *does* support that, then the USB floppy can appear as A: or B: for anything which uses the interfaces the BIOS's compatibility mode is providing.

If that isn't enough, then you need to search up how to slipstream stuff to build a custom XP install disc with the driver you need already present.