This notice will occur on most Yamaha keyboards at one time or another. This usually happens when you are loading in something that is not meant for the keyboard or is corrupt or even to large.
It is the keyboards way of telling you you are doing something the keyboard dose not like. No need for panic just do as it asks Power off and after about 10 seconds power on. Get rid of any dodgy styles or files that have caused this.
I agree, the warnings are for this ... Unfortunately in my case there are two alternatives
1) a failure of the SX900 (out of order)
2) a bug that caused the boot screen to "freeze" (I tried everything).
The problem occurred after putting and removing the maximum of the favorites, it happened to another user, always with the favorites (Blackpool).
For this reason I thought more of a bug than a failure. Returning to this post ... I would say to be calm as EileenL says.
it is just a warning of incorrect reading/writing.