A Cold Wallet is probably the most secure way to store crypto currencies. Cold wallet is an umbrella term, because it describes all wallets that are stored offline. Whether the wallet is in paper form (Paperwallet) or offline on an external hard disk is irrelevant.
But since it is not connected to the Internet, it is of course impossible to access it via the Internet. The risk here is however that for example with a theft of the non removable disk or with a fire the assets of the crypto currency (Bitcoin, Ethereum, Litecoin etc.) are also away. But this risk is rather small, therefore Cold Wallets are considered as very safe.
Note: You never have 100% certainty! It behaves exactly the same as with money, there is also no 100% certainty!
