[no subject]

> It would be interesting to write sectors with a longer (internal)
> checksum, then perturb head loading, speed, and alignment and see
> what fraction of "good reads" resulted in corrupted data.  My purely
> anecdotal evidence is that this is quite rare.

The XOR checksum is only 6 bits per 342 nibbles. Quite non-linear for a 
simple analysis, including the sequencer rule for MSB=1, but obviously 
very little in terms of data excess.

There's also the sparse nature of the nibble -> 6-bit decode table and 
whatever is stuffing the "invalid" but possible 128-64=64 entries is just 
making this more mind-boggling. :-) IIRC, although the table might have 
indication for incorrect nibbles in the stuffing, the tight loop of RWTS 
cannot have the luxury of checking it.

> Come to think of it, the decades-long experience of Apple II users,
> in which the Disk ][ performed as a robust storage medium, confirms
> that the one-byte checksum, plus field delimiter checks, delivered
> excellent reliability at very low computational cost.  If this had
> not been the case, the Apple II would have acquired the reputation
> of an unreliable computing platform.

My point was totally from archival point of view - the uncertainty that an 
image is really done "right" by automated means when the media is 30+ 
years old and close to extinction. Even if one has two or more physical 
copies of the media, they're possibly distinct enough to be useful for a 
naive bit comparison.