Thanks for your responses, i can say that:

first problem (usb disk busy) is reported on troubleshooting list as a compatibility related issue, with the suggestion of make a format of the usb stick.

Surely the problem is VERY annoying, i have the doubt that, if you have recalled last song from USB, periodically the OS could update that link and if in the meanwhile there is Usb recording activity you experience a stop.

Furthermore i have experienced this error message more in "live" recording rather than when i recorded the play of a midi song.

Probably, if you are not in live contest, it could be better to record in the sequencer and, when you have a good song saved, transfer it to usb stick.

But if you are in live contest and you are also recording voice a general stability of usb recording is required.

I will investigate compatibility issues but it happens to me with two different sticks, for now.

The slowdown problem seemed a cpu overload (never experienced before), i don't think to have gone over the poliphony (and in this case i should expect to ear less things but at correct tempo) but i was making some things, play style with full accompaniment, solo and echo, recording it in sequencer.

Not a "strange" activity, i have to consider that usb stick (inactive) was in and it has inside a large number of files,perhaps OS could have been in an indexing activity.

I will make other tries, no one have experienced slowdowns in the tempo?
I think that cpu overload could be a cause but i was worried if it could be an issue of *my* keyboard or all yamaha keyboards uder particular conditions.

Could you suggest me some places to dowload styles optimized (or not) for the S900? Can i load styles made for Tyros 1/2/psr3K?

Thank you for your attention

Fabio