Welcome to Yamaha Synth.

You seem to be under several misconceptions. While both the Motif ES and S90 ES share the Yamaha logo and ES suffix, there is enough difference for them to be in completely separate product lines. Since you are interested in converting the "Song setup" you do realize one of the differences is the S90 ES has no SONG mode. That ought to give you your first clue that they are very different.

You are equating the Motif series' MIXING mode with the S-series' MULTI mode. The Motif ES calls Voices from 6 Preset Banks and 2 User Banks, while the S90 ES calls Voices from 4 Preset Banks and 2 User Banks... That would be a key reason why a Motif ES MIXING setup will not directly transfer to the S90 ES.

The Motif ES has 256 Voice locations not found in the S90 ES - this fact makes it problematical to use things like Program Changes - Voices are NOT in the same locations - and given the two years between products, Voices are bound to be different. They are simply going to be too different and not in the same locations. Therefore things like PERFORMANCE data will not translate.

The S90 ES has some 228MB Wave ROM, the Motif ES has 175MB - so they do *not* share the same Wave ROM. That just is not true.

That said, There is Voice compatibility between the S90 ES (2005) and the earlier Motif ES (2003), such that the S90 ES can load VOICE Libraries made for the Motif ES, Motif (classic) and the S90. There are built-in "look up" tables so that when these earlier product's files are encountered by the S90 ES, it will be able to find the exact Waveform or a suitable substitute Waveform. This "look-up" table means, that Yamaha actually *did* give a lot of thought to this. That forethought means that S90 ES owners were able to load the previous models with a similar Voice architecture... Because of the differences in Effects, Filters, and Envelope Generators the sound may be slightly different. But to ask why didn't Yamaha think about this is completely wrong.

"Voice compatibility" goes so that the newer product can load the previous, never the other way round. So Voice compatibility is not "vice versa". The newer can load the older... Like time it progresses forward.

The Motif-series are designed for those requiring Sampling and Sequencing integrated into a music production instrument.
The S-series are designed for those interested in the Motif VOICE Library, but do not require Sampling and a recording Sequencer on board. I don't think much time was spent "thinking" about customers moving from one to the other. Why? They are clearly designed for two different customers. (Did you purchase the wrong one?)

That said, the S90 ES can directly open Motif ES files, Motif files, and S90 files, and it can load the Voice data.

Hope that helps and clarifies, a bit what you *can* expect to work from your Motif ES files.

Thanks Bad Mister!
I put this question hoping to help an Italian Motif ES user that wanted to share his setups with a 90ES owner. As you already pointed out the two instruments can share just Voices data but not Performances one. This is somewhat annoying but that's it.
Being a Motif XF and S90ES owner I won't face this problem because the two instruments are pretty different from each other.