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There is one interesting new drive Blaustahl USB, that is based on FRAM (Ferroelectric RAM) that could last up to 200 years, but it can only store 8kb of data, that means only text files.
It is expected that future firmware updates are going to include encryption for better security.
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But in terms of lifespan, M-Disc would be better option as it claims can last 1000 years with bigger storage. Although you need to encrypt the file manually and certain DVD writer which also support M-Disc writing.
USB flash drives generally don't have good lifetime unless they are industry graded, so I would not trust them as only backup option for storing bitcoin keys.
I'm not sure why the OP isn't considering USB external hard drives for backup purposes, because HDDs have a much better lifespan than the flash that you can find in a USB pen drive.
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You're overthinking. Have you seen OP's feedback or post history?