Date   : Fri, 30 Jul 2010 21:11:06 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

> > P.S. I tried to find a manual or something to confirm the existence of 
> > a DDFS offering 720KB, but surprisingly failed
> 
> You won't find it. Maths again. You'll need nine sectors per track for 
> eighty tracks on two sides. Oh, and 512 byte sectors. 

I'm fairly certain Watford DDFS uses 18 256-byte sectors per track. I suspect
they had to reduce inter-sector gaps though, which makes it 'non standard'
and in theory could have problems with some FDCs, though I don't think the
177x had any trouble.

That's also why ADFS on the Arc + Master 512 discs use 5 1024-byte sectors 
instead of 10 512-byte sectors - few sectors mean few inter-sector gaps!
Again, like with DDFS, I've seen the latter done (on a LOT of Atari ST discs),
but it was never officially supported outside of custom formatters.

Tom