I have built up a substantial collection of MP3 files -- most of which were ripped from CDs that I no longer have (I had to sell off most of my CD collection in response to a financial crisis). Since these are now my only copies of music I paid a lot of money for back in the day, you can see why I'd like to make sure I'm backing them up securely. If I can't pay the monthly fees for cloud backup of the whole collection, I'd like to at least use the longest-lasting, most durable physical media possible. I'm currently backing up to an external HD, but I'm wondering whether one or more flash drives would prove more reliable over time. Recordable optical discs supposedly have a limited shelf life, though I have some that still work fine after 10+ years. Opinions?