Are USB or flash drive considered flash memory? I'm also using it as a form of backup to my other wallets. Is it safe to use those even if you connect it to your computer which is connected online? Engraving it on a metal will just be the same to my mother, if she sees it somewhere in our house then she could throw it away because she doesn't have any idea what it is.
USB or flash drive can be corrupted, also if you connect it to an online computer, it's really not safe since you're increase the chance of getting malware, virus etc that would access your private key. Even adding extra security by compressing the file and using password didn't help, a hacker can easily find the vulnerability.