[mattpharr]

So -20480 works out to be 0xAFFF.

My trusty copy of "What's Where in the Apple", which includes a catalog of where everything is in memory, says that the routine to write the VTOC from its in-memory buffer to disk starts at $AFF8.

The VTOC is the volume table of contents (https://en.wikipedia.org/wiki/Talk%3AApple_DOS#Volume_Table_...), so calling into the middle of that presumably ends up overwriting the VTOC with junk, which would indeed make the disk useless.