I remember back then, there were few people that used to drill an extra hole on them to double their capacity.
Surprisingly, it worked!
While the other way around is plausible I am quite unsure of the reliability of such transformation in the long term.
1440 KiB floppies were probably more expensive not only because they had an extra hole, rather, a different material was used like in CrO2 or Metal tapes.
Is this reasoning correct?
Standard and double density disks used an iron oxide coating. High density floppies used a cobalt coating.
The benefit of the cobalt coating was that bits could be placed closer together, allowing for more bits per track.
Some confusion arose because the only difference between standard and double density disks is that SD uses FM coding, and DD uses MFM coding. MFM allows bits to be packed closer together without changing the magnetic properties of the disk, so it was possible to use disks designed for standard density without any problem.
Since HD uses a different coating to get higher bit density a DD disk may work but would not be considered reliable. In particular it may have issues with certain drives, or not be readable when used in a different drive to the one it was written on.
This worked... for a while... but eventually the low density disks written with high density data can start to develop data errors and become corrupted and unreadable.
The way that magnetic media works is that it has a coating that can retain a magnetic field that has been imprinted on it, and it has a resistance to demagnetization / remagnetization, known as coercitvity.
Magnetic media needs to be able to resist casual demagnetization by the Earth's magnetic field, from other magnetic media placed near it, but most importantly from other data written directly onto the media.
On most disks there is usually up to a full track width of space between adjacent tracks, because in addition to writing an imprint of a magnetic field directly underneath the drive head, there will be an additional unfocused fringe of a magnetic field around the edges of the drive head. If the tracks are space too closely together, this fringing can partially overwrite the magnetic patterns of adjacent tracks.
For a high density disk, the media has a much higher coercivity. A low-density drive will not be able to write to these disks because its drive head does not produce a strong enough magnetic field.
For a low density disk written with a high density drive head, the signal intensity is stronger but also smaller, but with also a stronger unfocused fringe around the drive head. This unfocused fringe has minimal effect on a high density disk, but for a low density disk it can be strong enough to change the media.
If this is happening, the high density data is slowly corrupted as the disk is filled with data over time.
Also because the data is more closely packed together on the media, it is possible for adjacent high intensity magnetic patterns to erase each other in a sort of tug of war on low density media.
For a bit pattern of 11011 the center zero bit is being stressed by the two outer 1 bits on either side with an opposing field. Over time the magnetic field in the center may weaken and disappear, becoming unreadable as 11#11, or it may flip entirely to match the two outer fields as 11111.
Temperature also has an effect on media coercive stability, so if these specially written disks are kept in a cool storage location, they are more likely to be readable later than if they are kept in a very hot attic for years.
Stumbled into this thing while cleaning up my basement. A density doubling disk notcher. A 3½ floppy is placed face down, turning the handle acts like a vise to press down the hole punch.
DISK WIZARD
Pat. pending
Made in Canada
My memory is that the HD floppies were made to a higher spec, so you could get away with formatting a lower density to HD, but at an increased risk of errors over time. However, Wikipedia disagrees with me:
The holes on the right side of a 3 1⁄2‑inch disk can be altered as to make some disk drives and operating systems treat the disk as one of higher or lower density, for bidirectional compatibility or economical reasons. Some computers, such as the PS/2 and Acorn Archimedes, ignored these holes altogether.
I remember being able to convert good quality DD floppies (Sony comes to mind) to HD floppies for MS-DOS and it worked very well and was cheap, but with low quality/brandless DD floppies you were in for a very bad surprise.
On the other hand I used the inverse trick to copy long-track/protected DD (original Amiga games) disks with a hardware copier onto HD disks with success.
It sounds like you're referring to the write-protect notch. Single sided disks had a notch on one side, allowing you to write on one side. You could cover the notch with tape to make the disk read-only.
Because of this, you could grab a pair of scissors or a hole-punch and cut a notch out of the opposite side, flip the disk over, and use the other side. The disks, fortunately, were symmetrical. I can speak from experience, having done this to hundreds of old floppies.
There was possibly a bit of risk to the procedure. The manufacturers sold double-sided disks, for a higher price, and warned that cutting the notch yourself was unreliable. Perhaps due to the quality of the disk, and the possibility of leak through from the other side? Or, due to the fact that flipping the disk resulted in it spinning the opposite direction, and the inner lining could cause wear? Regardless, all the poor teenagers I knew did it to all their disks.
