What would cause a regular pattern of bad sectors?

[Chuck Guzis]

On 04/24/2017 06:51 AM, dwight via cctalk wrote:

> Many format programs don't look at the index anymore as it isn't
> important for reading the disk. It is only used to indicate that the
> disk is turning.

That may be true for some systems, but for the NEC 765-based controllers
found in PCs, the format operation always starts at the index (and ends
there too).  What isn't dependent on the index is the sector ordering;
it's not at all uncommon to see a cylinder- and track- skew factor built
in, such that cylinder 0, side 0 starts with sector numbering
1,2,3...18; cylinder  0, side 1 starts as 4,5,6...18,1,2,3 and so  on.
 It avoids a lost rev when moving from track to track.

Similarly, when reading, if the IDAM for a desired sector isn't seen
after 2 index pulses, the 765 calls it a "not found".

There are index-ignorant controllers out there, but the PC isn't one of
those.

My guess on the symptoms described is a scratch or spill.  Sometimes
these floppies can be brought back by a simple cleaning of the cookie.

--Chuck