I'm planning to (finally) backup 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?

As far as I'm informed every device that's omehow connectes is potentially vulnerable.

Would this be an acceptable solution?

Having 2 HDD's, one conected to Pi as a mini NAS, one completely offline. Save any progress from my Live System to the Pi periodically. Let's say daily. So I have a daily Baclup of my Important Data. I also could use a 64 GB SD Card, as this is not a huge ammount.

Also, I create another Backup manually every x Days/Weeks to the standalone Device. If the Pi is also corrupted I will notice this and still have a (not up to date but working) Backup.