3.5" floppy disks

3.5" disks were the most popular diskettes over time. Introduced as a portable version of 5.25" media, they were more durable and finally could fit more data. They were used in PCs to late 2000s.
They were developed, with the most popular format holding 1440kB, but Amiga and early PCs used lower-density 720K. Some rare drives can use special ED (Extended density) 2.88MB disks.
Typical 1.44MB disk can be formatted to larger capacity (DMF - about 1.7MB).

Even today (2012) some mainboards have floppy controller built in, but these are rare now.

Manufacturer: different

Type: Disks
Capacity:  360-2880kB

 Disks should be stored vertically in a box. Leaving them horizontally one on another is not a good solution. Some disks had plastic transparent foil sleeves in which they could be stored for longer time, but they aren't popular now. Before inserting disk to the drive sleeve should be removed,  if not, drive will be jammed.

If you try to use higher density floppies in lower density drives (like PC disks in Amiga), you may have problem: Data written in higher density drive will be usually readable, but erasing them and writing data in 720K drive may cause errors.
Why? Higher density drive has stronger head, writing bits stronger. Weaker head can't remagnetize higher magnetized surface.
There's only one solution: To use 1.44MB drive formatted disks only to put data TO low-density drive system, but write on this system to completely magnetically blank disks (after formatting in low-density drive), these disks should be never written with a higher density disk drive.
Making these blank disks is easy: Just demagnetize the disk entirely. To do it, you can use AC electromagnetic coil or even transformer soldering iron. Standing 2-3m away of media power electromagnet or soldering iron on. Slowly move it near the disk, make few swinging/round moves few centimeters near media. Repeat it for about 30 seconds, may be longer, but DON'T TURN IT OFF. Before turning off move coil/iron 2-3m away from media, or it'll stay magnetized some way.

In general, older disks of good manufacturers may be better and may have less errors than newest, as manufacturers decided to push floppies away from market as fast as possible by manufacturing poor media.

To read a PC disk to a computer for archiving purposes, you should use an old PC with DOS (Norton Commander's "Copy Disk->To Image" or RawRead), Windows 9x (WinImage or RawRead) or even Linux (dd utility, think before you type!), but avoid using WinXP and later systems. These systems have very poor floppy disk drive handling routines, probably to make users not use floppy disks and buy USB flash drives. This will give you many problems, false read errors and other things like this. USB floppy drives are even worse - they are slower and won't read old disks reliably. Forget about 720K or XDF, even DMF format with USB drives - most of these won't read it, some may even damage disks.
Apple computers may use USB drives (tested in Mac OS X 10.4 and my PowerPC G4 MDD), but not for lower-density disks - These disks may become irreversibly damaged instantly after inserting to USB floppy drive, even with write-protection tab set! Of course floppy routines in Mac OS X 10.4 are even slower than in WinXP. Many times you'll get few "tries" to read a disk every second, this is completely normal thing. Unmounting a disk 2 times is quite normal.

As I put a few downloads with Teac FDDs in 5.25" drives page, here's Teac for 3.5:
 - Let's start with the same knowledge base backup.
 - FD235J-3631, FD235J-3653, FD235J-5631 and FD235J-5637
 - FD235HF-3xxx thru FD235HF-Axxx Series
 - FD235HF Series (General specification and installation guide)
 - FD235HF Series (Technical differences between FD235HF7/8xxx to Axxx and FD235HFAxxx to Bxxx)
 - FD235HF Series (Technical differences between models FD235HFA/Bxxx to Cxxx)
OK, let's talk about technical details. 5.25" drives used different technologies, 3.5" drives are very similar to each other. They all (except some early models) use worm-screw shaft and stepper motor to move head assembly. To spin disk, a motor installed in PCB is used (see photo). Optical bariers are used to detect end positions of head assembly, but to detect write protection tab mechanical switches are used.
In electronics, you'll find a standard drive number jumpers (always set to D1 in all PC drives), sometimes READY/Disk Change jumper. More jumpers are used only in earlier models.
[2019] On the left, we can see an interesting exception from the rule. This drive, manufactured by Nec in early 1990s, is very thin - like 3 disks stacked on each other. It was used in some notebooks and thin clients first, later units got a typical 3.5" brackets and were used in PC. Because in such small space it is just impossible to put a stepper motor for head assembly, this unit uses a voice coil mechanism like in some HDDs, but mounted right to printed circuit board. As this mechanism has no transmission, it is really silent and it's not easy to hear head moving in this drive.
Typical 3.5" drive - front view Head assembly, disk inserted PCB and motor assembly.
Early 3.5" drive - it's a bit higher Motor and metal tape mechanism More electronics than in later drives.
Head moving assembly detail, stepper motor and metal tape can be seen. Small, Notebook drive, from Zenith notebook computer. Everything is smaller in it. (small cyilnder on the left is stepper motor.
Modern USB floppy drive External FDD for IBM ThinkPad notebook computer. Difference between 1.44M and 720K (left hole)
External/Internal drive for Rocky II industrial ruggedized notebook. With rubber protection it can be waterproof. External/internal FDD for Omnibook 2100ct notebook.
Omnibook drive can be connected to computer's parallel port (onmy Omnibooks have this port modified). 3.5" internal drive for IBM ThinkPad notebook computers. There were 1.44M and 2.88MB models, this one is 2.88MB. It could be installed in the same bay as CD-ROM drive.