You move the drive head to a specific track by passing an ioexttd to the
device with td_seek or etd_seek set in io_command, and io_offset set to
the byte offset of the track to which the seek is to occur.

    DiskIO->iotd_Req.io_Offset = (ULONG)(TRACK_SIZE * track);
    DiskIO->iotd_Req.io_Command = TD_SEEK;
    DoIO((struct IORequest *)DiskIO);

   Seeking is not Reading.
   -----------------------
   td_seek and etd_seek do not verify their position until the next
   read. That is, they only move the heads; they do not actually read
   any data.