Shout-out to the people having this problem (including the future me), here’s the situation: you take pictures using a Sony Mavica (I use an FD75), you can view them in-camera, but when you try to download them to a PC they show up as corrupted.
I thought it was a Windows 10 problem, I tried with XP to no avail, same with macOS and OpenSUSE. There was no way I could open the files but using Linux I was able to see these:
It looked like something related to bad formatting or something similar, so I dumped the floppy using dd and ran a dosfsck. It tried to fix most of the errors: long file names, sparse files, truncating things… honestly, I don’t really know.
Once the image was fixed, I dumped it onto another floppy, inserted it into the camera and—well, the pictures were there… but I still couldn’t load them on any computer, so the image was probably still trash.
The Mavica has a copydisk function that copies a floppy; apparently, it has enough space to hold the entire disk in memory and then write it onto another one. But… I found out it does a raw copy, so I was just cloning trash again.
The solution was to select the photos one by one and use Mavica’s copy function, which works similarly to copydisk but copies individual files onto another floppy instead of the whole (corrupted) disk. Finally, the photos were accessible from every computer.
Notes on formatting: formatting a floppy with Windows doesn’t cause any weird behavior with disk geometry. The floppy I was having issues with was formatted in msdos using a Mac. Of course, you can also format directly in-camera (which is probably the best option at this point). After the recovery, I also tried reformatting it using OpenSUSE, which made things even worse—the floppy became completely unreadable, to the point where Dolphin kept crashing. I had to reformat it in-camera, which took a bit, but worked like a charm.