The ROCK OS drive is never used for storage (or anything else except the OS) which is why the build tips suggest using the smallest drive for the OS drive.
So, you really have 240 ish GB on your storage drive. Personally, I never go below 10% on any drive.
Yes. There was another thread about it if you are curious reference: (While the comment is about the internal OS drive and an external storage drive, the comment stands. you will not get individual drive size reports with the Network Connected Share)
But, frankly, given it is just the music stored on the drive; an external USB 3.0 device, works great without lag. Its cheap enough to test out. Get an external WD 4 tb passport drive and test it out. They are about 105. usd at the moment on NewEgg. You probably won’t notice a difference. Even if you do, you can use it to keep a backup.