Hi,
I think that if there is an accidental deletion, corruption via mechanical failure or virus, etc., to your "working" drive and you are unaware of this process, what is going to be the effect when you pop your second raid 1 drive back into the NAS? Is the NAS somehow going to miraculously know how to handle the drive discrepancies. You are assuming, that there will be only safe stable data added to your working drive. I'm not sure it would work that way 100%, plus how up to date is your "backup" data, 3-4 months at best. I don't know how important your data is to you. Here is the thing, Raid 1 basically protects you from a single hard drive failure and that is just about it. Good luck