Anything is possible, but Yamaha does not think there is a problem
And after thinking about it, there are two, maybe three reported cases from PSR Tutorial, of the SX900 freezing at the logo screen. So perhaps it's not really an issue at all.

It sounds more like a few isolated instances. If this was a real problem, with numerous complaints, I would think Yamaha would want to quickly find a software fix, rather than have to deal with returns and replacements.

There's no telling what might go wrong with a kb or why ...
A few years ago I was experiencing a 'lock-up' with my Pa900 (hmmm, maybe it's something about the '900'number) ... I got in touch with KORG USA and they helped me track it down to when I was searching for files on the USB ... I believe I sent them a copy of the USB files, and there was a corrupted non-KORG file on the USB that was causing the problem ...

Hopefully, YAMAHA will be able to replicate the problem and find a solution ...

This whole Freeze Up/Favorite issue may have been overblown.

Of all the forums and sites I checked, three people had this problem. One of them sent his back to the dealer and received a replacement. A second had his freeze-up fixed with the latest update, and a third is still working on his.

This is not to say, contacting Yamaha didn't help and who knows, maybe that led to the latest update that included the Favorites Freeze fix.

So like everything else, just relax and enjoy, nothing's perfect, things break, people pass. It happens

Ditto Larry! Totally agree! Love my SX900!