[Coco] Update on the 3.5" Coco drive and typ issue

[Robert Gault]

OK, after spending several hours trying to find questionable code in rb1773 I 
could not find any. There most definitely are incorrect comments about tpi 48, 
96, and 135 but the code works as it should.

So why was I having problems? After many tries at formatting a 3.5" disk using 
both my Coco3 and VCC, I found that both the Coco3 and my PC via VCC would 
intermittently corrupt disks.
Repeated formats on the Coco3 with either typ=$21 or typ$20 eventually produced 
a good disk. The disk when filled on the Coco read and wrote correctly; no byte 
corruption. However, attempts at backup to a 3.5" disk using VCC did frequently 
corrupt the disk. When that happened VCC was showing Trk11 Sec242 which is nonsense.

About the only reasonable conclusion I can draw from this is that the 3.5" disk 
might be marginal or that the timing constants in NitrOS-9 Level2 are borderline 
for 3.5" disks. As with any intermittent, you can't generally make any progress 
until failure becomes the norm.

This whole episode resulted from trying to solve a problem someone else had 
using a 3.5" drive on a Coco3. VCC was used to create a 3.5" 80T DS boot disk 
but that disk would not work on the Coco3 system for no reason we could find. It 
could well be related to the problems described above.

PS There are other ways to transfer disk images to real disks beside using VCC. 
One way is to use Drivewire to format Coco disks and generate content with 
scripts, backup, or dsave.
You do need to either have an DW HDBDOS ROM or some way of LOADMing or CLOADMing 
the ROM code on the Coco.