I don't trust flash drives, hard drives, or any external storage device of that nature because they can get damaged and the files inside will corrupt, including your seed phrase and other valuable documents you saved in there, but saving in a flash will be a bit better than leaving it online in an email or so because anything connected to the internet is very risky and can easily get hacked.

Writing your seed phrase down in a special book is more secure and preferable than using an external flash drive because it will be there and will not corrupt, but if someone has access to it, your coin is gone, because unlike flash drives, you cannot put a password in a book unless you use some encrypted words that only you can interpret, and you can also duplicate the papers because of natural disasters like fire outbreaks, water, and so on.