I'm planning to Backup (finally) all my personal Files from my Home Network on a new HDD.

If I'd use an external HDD with USB cable the ransomware would theoretically encript these data too. Would this be the case If I set change the writing permissions?

Alternative Question; how can I protect the external HDD against unauthorized access (including encrypt software).

Would a sperate linux PC (e.g RasPi with the HDD) be an alternative, if I write a program that copies all relevant files periodically to the Pi? If only selected files/folders would be transfered, the malware shouldn't be sucessfull in this case; would it?