John B. Matthews wrote:
I've been sent an apparently valid disk image that uses block numbers above 128. It appears that such values wrap around, storing data in track 0-2.Might this have been a patch to get more storage on a data disk? I recall tinkering with Apple's p-system BIOS and formatter utility when 40-track drives became common.
That's exactly what it is. The Microsoft Premium Softcard IIe (and perhaps others) had a special "data only" format that wrapped to the first tracks in that manner to provide more capacity.