so there is still data corruption occurring on the Thinkpad. only this time, write operations are affected. it's more or less random but much more frequent than the read issue i fixed earlier.
an old data word is getting counted twice. logic analyzer time. 😑
the data ready handshaking flag is getting cleared in the wrong spot. test1 is the flag, and the green M1 cursor shows that it is getting cleared even though DMA hasn't started
i was using a version of the "dma running" signal that was latched on the falling edge of CMD, so it was possible to trigger this clear signal even when DMA wasn't actually in progress.
my DIFDIAG program can see the drive and talk to it, but some functions don't seem to be working. this will take some troubleshooting.