Paper wallet i think is the most secured solution.

External hard drive, USB,  or any same alternative can be hacked today or tomorrow ...

If you want keep your wallet on your OS, there no 100% secured OS and all can be affected by ransomware attack ... even if your OS is 100% secure (Which is impossible), programs you install on it like: Adobe products, Office, and any needed softwares for your work are not safe !!! Each time new hacker discover new bug and an army of Cryptocurrencies hunters begin to exploit it actively  ...