It may not be a product-specific question, but I want your advice on offline backup.
Apart from daily auto back-up to two SSDs at home and sync with the public cloud, I want to keep offline storage (data transfer by cable) as extra security.
Where would you store disks offline securely?
I’d also consider taking that offline backup (USB based disk or the like) to separate physical location that is separated from the machine by a distance that is measured in kilometres (or miles) and not metres (or feet). A friend in the next town, parents house, etc.
Taking into account risks (water damage, power surge), I guess offline backup is still valid for data security, right?
Yep, you should consider the various threats and the ways to mitigate them with threat modelling.
Accessing the backup and restoring the necessary data in a fixed time can also become a problem.