Then I sent 40 sat to a receiving address of this last cold computer hoping that the amount would appear on the first hot computer. The transaction is confirmed by bitcoin explorer, but nothing appears on the hot wallet and I wonder why.
It was revealed that you used 40k Sats to test.
Why people don't use Testnet for cheap testing? It's free, there are free faucets like
https://testnet.coinfaucet.eu which give you near instant enough Testnet coins to play with, usually more than 1mio tSats on one request from the faucet. That is enough to play around with your wallet and practice a bunch of things to get very familiar with your setup. All without fear of loosing anything of value (well, show some respect to those cute tSats).
I'd say, if your wallet doesn't support Testnet coins, ditch the wallet and chose a more decent one. Anyway, Electrum supports Testnet Bitcoin with ease. If you can do recovery with Testnet coins, your recovery shouldn't fail in Mainnet as long as you don't mess up with derivation paths.
So, it was the derivation path in the end it seems. Finally...
What I want to say and advise: practice thoroughly every aspect of your wallet setup including full desaster recovery before you load your wallet with coins of value. Testnet coins are perfect for this, except you can't send them to exchanges or similar.
How many wallet users actually do such thorough exercises and practice with their wallet setup? I'm not sure, if I want to hear the answers.
Of course it's imperative to use a secure and airgapped/cold environment as testing playground. You don't want to spoil the security of your recovery words, in particular if they are coming from a hardware wallet.
Document redundantly your recovery words together with some additional helpful details like
- date of creation
- which wallet generated the recovery words
- purpose of this wallet
- derivation path and if more than the first account is in use
- a subtle hint (more hidden than explicit) if mnemonic passphrase(s) are used, but document those in any case separately, never together with the mnemonic recovery words
