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 
Keep this in mind. Make sure to use a long unique password for the wallet and be sure to have multiple backups.
For added security you could put the wallet in an encrypted container as well.
The key here is multiple backups.
You wouldn't want a dodgy usb drive to be the reason you lost your bitcoin.
With Electrum I'm under the impression that I don't have to use any backups at all as long as I have my seed.
Not 100% sure on that as I don't use electrum, I always use bitcoin qt, never had any bitcoins stolen from it and just put a heavy encryption on it, I think a lot of people now go over the top with security just don't do anything silly and you'll be pretty safe, no viewing unchecked websites or dodgie downloads and your pretty much secured.