In the TRS-80 thread Robbbert mentions that
I've had problems using wd_fdc as well. In my case (2 different drivers), firstly 0xd0 is sent (which seems to work), followed by 0xc0, which causes the fdc to stop responding, returning status 0x81 forever. I've been patiently waiting for the author of the code to fix it, but nothing so far.
I should point out that using WD1772 works fine, but 1793/2793 etc does not.
... the data sheet mentions a fundamental difference in the xxx3 models to the other chips, so I thought this might help