whit3rd Inscribed thus:
The 'external USB floppy' consists of a case, USB interface, and
floppy drive. Some such devices have a standard floppy drive
34-wire header connector and you can connect those to a
5.25" mechanism. Usually, USB drives are bus-powered
and take +5V only, while the 5.25" unit will likely need +12V as
well.
I assumed that the OP would realise that he would need an external power
source. I just used one of the spare HDD plugs on the desktop machine.
So, the really important part of the 3.5" USB drive is just its
interface component; you'll need a 5.25" drive, a case, a
data cable, and a suitable power supply, to complete the unit.
Thats correct ! I didn't bother with a case since it was very much
experimental and done just to see if it would work, which it does.
The first "Laptop" USB floppy drive I examined had one of those slimline
floppy drives using a thin flexible ribbon between the electronics and
the drive. Which is why I gave the warning.