Korg Poly800/EX800 Users

hi, i've made many back ups of my poly-800 mk-i on tape, with different 
cassete types, different tape recorders an in pc, even at 48khz 24bits, 
and when i do a verify and displays GOOD.
then, i modify some patches and save the changes, and when i want to 
roll back to the saved tape, it's says that the tape it's not ok.
that is pissing me off, because sometimes, if you unplug the power 
cord, the  87 and 88 patches loses his data.
i don´t know why this could be hapennig. the last time i've been 2 days 
programming all the parameter of every patch with the factory chart, 
cause the loaded tape erased all my presets :S


any ideas?