I think at least get 2 ba but not all in one time. But than will nas got virus one? Lol. What if electric shortage?
There is no need to get 2 hard disk for hot swap, because you will not likely be in time to prevent the next hard disk failure if your components are so fragile.
Lets be concise between backup and hot swap. Backup is just another copy of the same data. Hotswap in storage normally refer to the availability of extra disk for automated swapping with the failed component in an array in the event of a disk failure. Without hot swap configured, it means the administrator will need to manually place and configure a replacement disk into the array.
No hard disk appointed for other purposes can be used for hot swap. Hot swap disk candidates cannot be repurpose until they are removed from the array configuration.
NAS do not provide protection against virus. It is the job of an antivirus job to detect virus. NAS that allow running of applications in it, may have antivirus packages offered to allow scanning of the array without the intervention of the user or any other systems/hosts. This is a feature, not necessarily available to all NAS solutions.
Electric shortage happens for all peripherals may it be computer related or even storage related. The common approach to using UPS to prevent such event from corrupting the storage system. Synology offers integration with some UPS where the UPS can notify the NAS in the event of a power failure. The NAS can be configured to react by shutdown as soon as possible or shutdown after some time or shutdown when the battery in the UPS is low. The main purpose is to allow proper shutdown of the NAS in the event of a power outrage. That will prevent any form of disk corruption. UPS also prevent against power surge, and spikes.
I have been running my NAS 24by7 for more than 2 years on RAID 6 with 2 level backups. Throughout the lifecycle, I have encountered 1 or 2 blackouts and 2 hard disk failures with NO data lost.