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.
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.
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