Yikes! Looks like the oxide came right off the media. Complete binder failure?

This makes me think of old 1970s and 1980s open reel tape media and how the binders often break down gummy after a surprisingly short period of time.

Cobalt (?) disk coating disgregated when I began to read the disk. Incredible. I accurately checked the two disks whole surface for dirt and dust before using them. The destroyed disk is a 1.44hd one, the second one is a 720k one and still working.

I had a box of 100 unusable floppies which caused that problem. They smelled bad and it looked like there was mould on them. To fix dirty floppy drive I used two brand new floppy disks, I sprayed isopropyl alcohol into first floppy disk surface and gave it a spin for a while in floppy drive. Then I wiped remaining alcohol with the second, "dry" disk.

This happens to me all the time !
I don't know what's up with the disks that do this, but there's nothing you can do about them unfortunately ...

And yes they do ruin perfectly working floppy disk drives. You have to clean the drive's heads after reading one of these disks or else, as you experienced, it'll damage good disks.

This is why we have to backup floppy disks asap, before they all start doing this ...

I found a similar case in a random ebay auction. Look at the floppy surface.

3.5" drives are much more difficult to clean than 5.25" ones. I opened one up to clean it and ended up destroying it.

Sometimes it's the drive that starts the process of eroding the disk. I've polished and buffed the heads in one of my drives with automotive wax and retensioned the spring for less head pressure when working with really old disks or rare disks that aren't likely to survive a 2nd read pass.

Basically the same thing began again today: this disk never moved since 1990!!! Looking at the surface, it had the weird white-ish pattern, mold or maybe the internal cleaning fabric that rotted away. Even the sliding metal shutter has signs of what looks like dried condensation. I tried to insert the disk and as soon as I heard the dreaded weird unusual sound, I removed the disk and I totally cleaned the front and rear surface with a cotton swab imbued with alcohol. Manually rotating the disk and gently repeating the procedure until most if not all the irregular patterns are gone. I noticed the more I was cleaning it, the easier the magnetic surface rotated and with less friction.

You need time and patience, I finally was able to dump all the data without errors! I risked when I inserted the disk at the beginning, that part is crucial, stop it immediately if your instinct tells something is wrong or you can destroy everything.

Good thing you managed to save that disk for one last read. Sadly sometimes disks show no sign of wear, I believe mold can cause this but it may not be the only thing causing this phenomenon

Other than the mold, I don't know why but in my personal experience, 720 disks are "stronger" against oxide coating flaking off, I had a lot more high density destroyed (bad quality?) disks during recent years.

Sometimes files or entire disks without visible damage just become unreadable (the OS says they are not formatted). Demagnetization.