I got an old computer from some friends, so I decided to wipe it and set it up as a home NAS running Windows 7. My plan is to have two external drives: one as a network-accessible, read/write device for sharing data across my home devices, and the second as a backup drive. I plan to use the built-in Windows 7 backup software to make regular incremental backups from the client-accessible drive to the second drive. I will use the default Windows Homegroup sharing.
My concern is security, specifically with respect to ransomware. I don't plan on sharing the backup drive directly over the network, however I can imagine a scenario where ransomware is brought into the network on a client computer, or even on the NAS itself, by someone using it as a workstation. If the ransomware encrypts the client-accessible drive, no big deal, I have the backup. But how can I guarantee that ransomware, or for that matter any user/program other than Windows Backup, won't be able to encrypt/attack the backup itself?
I found this question, but the answers seem to rely on piece of third-party software or another. I'd like to stick with the built-in features of Windows 7, if possible.