I don’t know the whole story. Maybe it was nitrate decomposition - I don’t believe the master audio reels were magnetic, I thought they were optical. I just know they were on film (tape didn’t exist yet).

The nitrate destruction thing is also something I can’t find the info on, the description of what happened (by David Gerstein) may have been on a message board or blog which is no longer around.

Also, it could have even been before Eisner came in, in the Ron Miller era or something. And some of it could have been due to decomposition, not just bean-counting. (One animator/historian saw some Disney-held nitrate prints of old B&W Mickeys in 1969, and I think he said they were starting to decompose even then?)

And I have no idea of anything Fantasound-related was tossed at the time. All I know is that B&W Mickey Mouse short titles had to be recreated for laserdisc in the 90s because the original title elements couldn’t be located in the Disney vaults, yet at least one of the cartoons turned up with original titles in the early years of the Disney Channel, suggesting they had access to elements in 1983 that were missing a decade later.

(Steve Hoffman’s separate claim that they disposed of original IB Tech backup positives going all the way up to 1974, which would have included stuff on SAFETY, is more troubling.)