[systemtest]

I would suggest always having one of the three disks in a seperate location. Never have the three disks physically close together, not even when swapping them out.

Also, have an offline backup that is not connected to power. Power surge at night and all your data is gone.

[mironathetin]

Well, yes and no. People who take their backup drives away from the computer tend to have - a completely outdated backup. There is no 100% safety. If power fails while the backup is done, well there is still the original disk. How high is the possibility that it crashes in the same moment?

[systemtest]

It talking about a power spike. If lightning strikes you could toast your computer and the connected backup drives. That is why offline storage is important.