A clarification: track format is same as 5.25" diskette, with exactly
the same ~50kbit capacity (at a fixed 300 RPM though).

So if the 400K is done with 50 tracks x 32 sectors, then it's
unusable. I am seeking for a way to extend the tracks only.

Current investigation (thanks to Bob-Sander Cederlof's "Apple Assembly
Line") shows the limit to be somewhere 50..63 tracks. I will further
consult DOS 3.3 disassembly until some knowledgeable pops in with the
answer.
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