Read a post on this from darthcoin, giving the exact same advice.

And I beg to differ, while I appreciate the educational value of it.

A proper hardware wallet is meant to do only two things

Absolutely...nothing...else.

And that is why it is a terrible idea to use a generic storage device to keep your keys, allow those keys move to any RAM (as they must, in case the signatures are being calculated by the host computer), and letting another host device read the contents of the USB drive.

Even thinking that another host machine (the computer where you plug in your ultra-secure flash drive) can run generic computation on the data on the drive? How crazy do you have to be?

So no, you should keep your keys in devices whose hardware allows nothing but signing events, not another generic memory stick providing the attack surface as big as a whale.

Notably, this is applicable for the bulk of your stash. A bit of fund for your short term spending needs in a soft wallet in your Android device? Sure, if you can do a bit of research on the app developer.

Build your own hardware wallet? Only if you are a hardware and firmware expert who can design and fabricate a chip _dedicated _for storing keys and signing transactions. Do not use mass produced general purpose memory stick as a proxy for a hardware signer.