I demo constantly Yamaha PSR S and now PSR SX series, and in my demos I use in different ways the Multipads, for a whole song sang by a friend or an a-cappella, or just fractions or a verse then a chorus....in separate Multipads, and ALWAYS I have my files in read in real time from the USB as long is 3.0 of course. I Demo mainly the Latino Expansion Packs. Sorry this link is in Spanish, I am using ONLY the styles from the Expansion packs I had installed in that PSR S975, is the vocal of a friend of mine Toño Alfaro, singing a popular Mexican song "De que manera te Olvido" is a Bolero Ranchero, as the initial style, then I ilustrated that you could change the genre of the song with different styles, I know I used to many, but is to give the idea of many possibilities, so, is a double demo, using the Audio Multipad and showing the versatility of the expansion packs. Here is the Link to that video....
https://www.youtube.com/watch?v=zc8WT16qBtQ&t=59s the song start at 1:33 I had to give the explanation I did just as above.
Long time ago, when I began demo this feature on the PSR S950 I had trouble because either on the playback of the multipad suddenly stop, or if I was audio recording, stopped by itself, etc....and that is when by process of elimination, found out the culprit, the USB 2.0 drive, way to Slow to read audio or recorded.....so, the solution was the USB drives 3.0....since then, NO problems ever again.
And no to be concern regarding running out of room, I consider myself a "heavy duty" user.....and the drives I use at least are 32 Gigs, I have 64 and even 128 Gigs...NO problems, and a great advice from a great computer teacher I had long time ago, told me, there are only TWO types of Storage (Floppy, SC USB, HD SSD, etc), the ones that Broke, and the ones that Will. Always make a Backup....in fact, make a Backup of the Backup...I have one in my Google Drive too.
My 2 Cents
Manuel