Single sided, double sided and flippy disks
The difference between a one-sided and a two-sided 8 inch disk can be easily recognized by the position of the index hole.
What is a Flippy Disk?
Floppy disks were expensive back then! Why not just flip a one-sided disk and use it on the other side too? Nothing easier than that. Just punch a second hole into the 8 inch disk and you're done. This is how the so-called flippy disks were created.
To the right the original index hole and to the left the homemade one. It is a bit smaller, but works fine in operation.
(Herb Johnson (hjohnson), VCF, March 17th, 2019)
8-inch (soft sectored) diskettes are physically either single-sided, or double-sided. The difference is NOT in the rotating media inside the sleeve; the difference is in the location of the index-hole in the SLEEVE. The sleeve's index hole is at a different location (rotary angle) for SS versus DS. But the rotating media (the cookie with the magnetic coating) is almost always the same for both; coated on both sides. And the magnetic coating is the same for single-density and double-density. So you can reformat between single-density and double-density (with controllers that can do that). Drives are NOT "single or double density"; they are single or double sided (one head or two heads).
Why doesn't matter much. But a 8" double sided drive, can 'tell' immediately if a diskette is single sided or double sided. Then it knows where the track starts (relative to the rotary angle of the rotating media). And an 8" single-sided drive, cannot operate a double-sided diskette (and goof up half the diskette).
Regarding "bulk erasers". Look up "bulk audio tape erasers". From the era of audio tape. They are good for scrubbing diskettes too. - Herb