Recovering data from an old Seagate ST-225 MFM HDD.
The video illustrates how (lightly!) rocking the drive helps with reading.
CH x,y OK means the program was able to read track x, head y without errors.
In case whole track reading fails, the program will try each sector, printing a dot "." for each sector.
If sector reading fails it prints an asterisk for each failed attempt and retries up to 10 times. If it doesn't succeed, it prints the "Error reading CHS x,y,z" where x is the track, y the head and z the sector that was not read correctly.
Many sectors on heads 1 and 2 have problems and can't be read without rocking the drive.