I just wanted to reply to this thread to give my experience. I have tried to do the very thing that the OP was intending: to find a way to hook up an existing floppy drive to the motherboard via some sort of USB bridge. I tried several ways:
1) A USB-to-26pin adapter PCB taken from a cheap USB floppy drive connected to a slimline drive that is part of an old CF/SD card reader (https://www.youtube.com/watch?v=91MblZpho1A).
2) The USB-to-26pin adapter from #1 connected to a 26pin-to-34pin adapter (http://r.ebay.com/24yLTm). This was then plugged into a real 3.5" floppy drive.
3) An eBay USB-to-34pin adapter (http://r.ebay.com/QMRv48) connected to a real 3.5" floppy drive.
All three methods gave the exact same results. They all worked as expected in terms of reading and writing floppies, but with one annoying caveat: once per second the drive light will flash and if there is no disk in the drive, the stepper motor will move back and forth one step. It gets annoying quick and the exercise on the stepper is not ideal. This happened with several different floppy drives from Teac, Mitsumi, and NEC. It happened regardless of the host OS (Windows 10, OS X, and Linux). It always behaved exactly the same, including the slimline drive that was part of my old CF/SD card reader.
However, when I used the slimline drive from the old USB floppy drive (from option #1) with any of the other adapters, it worked great. So I ended up swapping the slimline drive out of the CF/SD card reader with the one from the USB floppy drive and used method #1. It's not ideal, since the reader is so old I have to keep my other card reader in there as well (the old one does not even support SDHC cards).
My theory is that there is some sort of polling going on by the USB bridge to check for the presence of a disk. I'm guessing that it power-cycles or resets the FDC front end, which makes the drive glitch. Probably some power saving feature. Note that it doesn't matter if I power the drive from the adapter or directly from the power supply. The only thing that stopped the glitching was to use a disk drive that is designed for it. I did not investigate if USB power management was somehow to blame.
Anyway, I hope that helps anyone who passes through here in the future.