The way the app currently accesses the USB drive is via Android's high-level storage access system, which should be the safest way of doing it, but it seems to have many device-specific quirks. I would hope that it's not possible for the app to corrupt a drive, but since it is leaving the actual hardware interfacing up to the Android operating system it's possible that there could be some weird bug at that end. I wouldn't have the power to fix that, though.
The app is sandboxed by the operating system so that it can only access files / drives that you explicitly permit it to access, so the app could not access your other SD card even if it tried. That's not to say that Samsung's version of Android does not have a bug in that area that could allow external storage paths to get mixed up. It's unlikely, but not impossible.
To get around the device-specific problems, I tried an alternative system where my app would directly read the USB storage at the hardware level, but it was less reliable and more likely to cause things like corruption so I decided against deploying it.
In any case, I think my brother has a Note 8 I should be able to borrow and test with, so I'll put that on my to-do list.