Personal feedback: You have a tendency to make obvious, unsubstantiated, often slightly incorrect statements with a very high level of confidence and assuming you're talking to an idiot. I'm not learning anything from you. Let's both move on. This is my last post in this thread.
If you do want to learn something yourself, I'd suggest, as a homework exercise, taking the time to architect this out, and running some numbers on things like:
- How often local devices fail (HDD AFR)
- How often remote devices fail
- Overhead of monitoring / catching each type of failure
- Time for synchronization
If you can, include algorithms along the lines of those in RAID5. I would also back-of-the-envelope cost estimates.
The goal of backups is to decrease odds of data loss to some manageable, quantifiable risk (e.g. from ≈1% AFR to e.g. ≈0.01% AFR), and what tolerable levels of risk are is an ROI and expected value calculation: odds of failure * cost of failure should be less than the cost of the backup system.
It's a good homework exercise, and purely for your benefit. E.g. that's the sort of question I might ask on an interview, and answers like yours would not lead to a hire decision. The people I least enjoy work with operate from best practices (often using strong language like "iron laws"), and can't operate from first principles.
In this case, this is all quantifiable.
You don't need to post your response. There's a reason I asked someone who might now, and you cutting in was a pure distraction. I'm done here.