I'm talking from personal experience, across multiple customers USB sticks, and multiple models and brands.
Formatting a USB stick (quick format on PC) is totally fine, yes, but on a Mac it can cause read issues. Also USB sticks with split drives, like the windows 2000 / ME standard also have issues; they show up on Yamaha OS as "This USB is unformatted" when in fact, it already is.
There *was* a size limit; the Fat16 Yamaha's like PSR3000 and CVP309 could only ready USB drives up to 2GB (there were later OS updates to allow Fat 32). The current Fat32, I don't know what the size limit of the USB is, only that the largest file is 4GB. I posted the list because I've encountered issues with 32,64 and 128GB usb sticks. I carry around a 128GB, and the PSRsx600 doesn't recognize it at all, and the CVP709 stalled when saving on it, but I've had absolutely no issues on any other board / brand.
But when I say formatting a stick on an instrument can cause issues, I'm speaking from (thousands) of customers' experience. For example, had a customer format a stick on a Tyros, turned out his Korg PA1000 could no longer read it. Things like that. And likewise, *some* flash drives work anywhere. One of the weirdest ones was Roland's TD15 drum kit; My USB stick worked on some but not all, we formatted another stick on one of the TD15's that *wouldn't* read my master stick, and it was no longer readable on the other TD15's that originally *did* read, but fortunately the PC could still read it. I've also had one formatted on a CVP709 that read on the 709 and all Yamaha keyboards, but wouldn't read on my PC at work (but it worked on the one at home).
That doesn't mean it guarantees it will no longer work on another instrument. But I've literally had dozens of customers who did format on an instrument, only to have it no longer read on another brand's, so I told them to purchase a new usb stick, and *not* format it anywhere, and that worked. It's fun to try the same midi files on a PA50, PA1000, and Tyros 5 for example to see what sounds best where!
My 32GB Kingston does read on the sx600, so I use that to port files onto the master 128GB Sandisk. I used to have a 64GB Kingston master USB (with OS updates, files etc for everything from Yamaha, Roland, Korg etc), but I pushed my luck on the data cascade issue (most USB sticks will corrupt once you pass a high percantage of used space) and it died completely.
This is across multiple sticks from approx 7000 customers and multiple work computers, I myself actually have about 40 USB sticks, including 3 Fat16's just in case I need to pull data from an old trade in keyboard.
If you check my YouTube channel, you'll see I film at work amongst dozens of models
And the reason I've never posted a Korg or Roland video? I *need* registrations!