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
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.