[14] What are the differences among single, double, quad, and high density floppy media? 

There is essentially no difference between single and double density media (if you can actually find any single density media anymore). Double density media has been tested and certified for double density recording, which is slightly more demanding than single density recording; conceivably, a dropout that might not be noticed in single density could be a problem in double density. In practice, though, I suspect that back in the days when manufacturers sold both DD and SD media, the media came off the same production line and the SD media would have met the DD standard nearly all the time. 

"Quad density" 5.25-inch media is certified for double density recording at 80 tracks per side rather than 40 tracks per side. This is quite different from "high density," discussed below. 80 TPS quad density media should work fine at 40 TPS double density. I've been told that there can be problems using 40 TPS double density media at 80 TPS quad density; I believe the person who told me this, but neither of us really knows the reason. All 3.5-inch media is certified for 80 TPS. 

You will also sometimes see media labelled as one-sided or two-sided. Both types have magnetic material on both sides, but on the disks advertised as one-sided, the second side may not have been tested. It's even possible, though unlikely, that the second side was tested and failed. 

Occasionally you'll find a "flippy" 5.25-inch disk. Such a disk can be recorded on both sides by removing it from the drive and flipping it over. It has two separate write-protect notches and two separate index hole cutouts in the jacket. Sometimes users would make their own flippy disks with a hole punch. 

There were also a few "flippy drives" made. These had two write-protect sensors and two index-hole sensors, and they could thus read and write on the reverse side of an ordinary diskette inserted into the drive upside-down. If you have disks written in a flippy drive but you don't have a flippy drive, the reverse side will be quite hard to read. See topic [26] for ways to do it. 

"High density" media is very different, and should not be interchanged with single/double/quad density media. It has different magnetic properties (higher coercivity, requiring a higher write current from the disk drive). If you try to format DD media as HD, the media will be too strongly magnetized and bleed-through can occur, garbling your data either immediately or after the disk has sat on the shelf for a while. Such a disk may also be difficult to erase and rewrite with new data. Similarly, formatting HD media as DD is not a good idea either; the media will be too weakly magnetized, and your data may be completely unreadable or may fade away after a while. These problems are worse on 5.25-inch disks than on 3.5-inch disks because the difference in coercivity is greater for 5.25-inch media, but they exist for both types of disk. 

You can distinguish between 3.5-inch HD and DD media by looking for a hole in the jacket on the opposite edge from the write-enable hole; HD media has the extra hole, while DD media does not. You cannot reliably distinguish between 5.25-inch HD and DD media if they are not labelled. There are a couple of ways of guessing: DD media usually has a hub reinforcing ring (but not always), while HD media generally does not. The color of the media also tends to be different; HD media is darker. 
