The initial back-up isn't the problem. It's keeping it current over time as you add new music. Then you need to keep track of which songs are on which drive and

The initial back-up isn't the problem. It's keeping it current over time as you add new music. Then you need to keep track of which songs are on which drive and also access your backups often enough that some of the routine error checking happens to mitigate bit rot. Especially a problem with flash storage since it really isn't designed for long-term storage without being plugged in.