It's not USB mass storage, but what you're describing sounds like MTP. It's the protocol used in a bunch of old (first-iPhone-era) media players, and I think t

It's not USB mass storage, but what you're describing sounds like MTP. It's the protocol used in a bunch of old (first-iPhone-era) media players, and I think the default for Android 5+ or so. Used to write software to interact with it on the computer side with the Microsoft Windows Media Player SDKs. It did have the whole VFS and send the data over then commit thing.