Sounds pretty secure, I just store mine in encrypted files and keep them in a few places. There's always a chance you make loose the USB drive and don't forget its not reccomended to use flash drives for long term storage. Pen drives degrade over time.... Apparently 
With this thought nothing would be stored on the USB drive. If it works the way I think it does all of the information I need to access the wallet is stored in the twelve word seed provided by Electrum. This way the only way I could lose the bitcoins here are through some sort of Linux vulnerability, an issue with Electrum, or something physically stealing my written down seed in real life.
Or maybe my ISP could do a man in the middle attack? Not that it's likely, but is that an issue as well? Since I'm downloading Electrum from a live internet connection. I guess thinking about it that way means that it could be my ISP, or Electrum's web server or anything inbetween...
Thinking that way you better avoid bitcoin all together
at some point somewhere there's always as small chance someone could get your coins, I personally wouldn't go to these extremes, if I where storing a mass amount of bitcoins and even then it would be split into smaller amounts among a lot of wallets, A man in the middle attack wouldn't be very effective as the address is generated on your side only block data comes through the isp,
There is no 100% sure way to keep bitcoins perfectly safe no matter what anyone says if you can access your bitcoins then so could someone else
I think a lot of people use Armory offline storage cold wallets and I think those are considered safe. Once they're in there as long as the computer isn't connected to the internet there shouldn't be anyway to access them.
I'm just thinking that my method is a bit easier and more flexible since I can get the coins back easily by restoring my wallet from my seed. But I also want to make sure I'm not making any big mistakes.