[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 People who take their backup drives away from the computer tend to have - a completely outdated backup.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.