I wonder whether "to save onto" comes from a past time, when the physical object (vinyl record, tape cassette, compact disc...) was more important. Now, data exists everywhere.
>> "Save the photo/document/audio TO/ON the computer/phone/camera/ipad"
To me, the same thing.
>> "Save the photo/document/audio TO/ON 'C:\Windows\' "
Here I would prefer "to", because 'C:\Windows\' is more like an address/place, and not a thing. In this case "on" sounds a bit weird to me.
I expect in 5 years' time, these meanings will have evolved further