Quote Originally Posted by SeijiSensei View Post
I use rsync every night to back up three cloud servers and my local server. Like you, I have a 4 TB outboard drive to which I write everything. It's big enough to store five days worth of backups, plus one from each of the past seven months. I wrote a bash script to do all this.

Using the --files-from and --exclude-from switches to rsync lets you define the directories you want to back up and those you want to exclude in simple text files.
Sorry for the late response, thank you!

Just to close the discussion, in case someone encounter the same issue, I tried to unplug a disk while the system was running: in this case the array was correctly moved to "degradated" state but it was still accessibile (even after a reboot). In the very end, I think the "problem" is the fact that mdadm needs to "record" the changes in the array and can not automagically detect what happened when the system was powered off (such as an unplugged disk).