This is good advice and people should follow it. Four failure modes sit inside it though, and each one has cost somebody their coins before. Checked against the vendors' own docs, quoted, because this is exactly the week to not take a stranger's word for it.
1. TAILS IS AMNESIC BY DEFAULT, AND THAT INCLUDES YOUR WALLET FILE
Tails' own documentation: "Everything you do disappears automatically when you shut down Tails."
That is the feature. It also means that unless you deliberately create Persistent Storage, the Electrum wallet FILE is gone at shutdown. Not corrupted, not hidden — gone. Your written seed words are then the only thing standing between you and your money, which is fine if they are correct and catastrophic if they are not.
Which leads directly to:
2. TEST THE RESTORE BEFORE YOU FUND IT
Write the words. Shut Tails down completely. Boot it again. Restore the wallet from only what you wrote on paper. Confirm the first receive address matches what you saw before.
Do that BEFORE sending anything, and again with a small test amount before sending the rest. Transcription errors are the single most common way people lose self-custodied coins, and they are silent — a wrong word gives you a perfectly valid wallet that simply is not yours. A restore test is fifteen minutes and it is the only thing that actually proves your backup works.
3. THE SEED ELECTRUM GIVES YOU IS NOT BIP39
Straight from the Electrum docs: "For these reasons, Electrum does not generate BIP39 seeds."
Electrum has used its own seed format since 2.0. Practically: those 12 words may not restore into a hardware wallet, a mobile wallet, or most other software later. It is a perfectly good seed — it is just not the standard one, and nothing on the screen tells you that while you are writing it down in a panic.
So label your backup. Literally write "ELECTRUM SEED" on the paper. Future-you, or whoever inherits this, will otherwise spend a very bad afternoon typing valid words into a wallet that keeps saying they are invalid.
4. VERIFY THE TAILS DOWNLOAD
You are downloading a security tool onto a machine you no longer fully trust, in order to fix a problem caused by trusting software. Tails publishes a verification step. Use it. An unverified image is the one attack that would make all of the above pointless, and it is the step everyone skips because it is boring.
AND ON THE PASSPHRASE IN YOUR BROTHER'S GUNSAFE
Right idea, one caveat: passphrase loss is total loss. There is no recovery, no support line, no "mostly right". If the gunsafe burns down or your brother moves house, the coins are gone as surely as if they were stolen — you have just replaced a theft risk with a single-point-of-failure risk.
So: more than one copy, in more than one place, and make sure someone knows the arrangement exists. The failure mode people actually hit is not an attacker finding the passphrase, it is themselves not finding it.
One more, if you are moving coins off a compromised Coldcard rather than starting fresh: send them to a NEW wallet, not into an existing one you already care about. The spend links the burned addresses to whatever wallet receives them, permanently and publicly. Consolidating within the compromised wallet costs you nothing — those addresses all came from one seed and were already linked — but the destination is written to the chain the moment you broadcast, and you do not get to take it back.
If you are still holding coins on an at-risk seed while you set this up, I published a small script that watches your own addresses and tells you if a spend from one shows up unconfirmed in the mempool. Addresses only, no seed, no xpub, no signing code:
https://blossom.primal.net/14e769afd58b606b3a5bc87d522bc278a63343f7c208e81e68bc4ffc30758507
Nothing legitimate ever needs your seed words to tell you whether you are affected. Firmware version and dice-roll count answer that. Anything asking for the phrase IS the attack.
