Keeping your keys on a USB drive is barely a secure solution. Even considering the short life span, your device can burn out due to improper disconnection or things like that. Write your key on paper, laminate it, put it in a safe, and that’s it. Maybe it’s not as cheap as buying a USB drive, even an expensive one, but there are small safes out there that are great for the price and do their job well. If you have a lot to lose then it should be worth it.