DDOS attacks block connection to the servers, they don't actually harm the data itself. You could probably overload a server to the point of it shutting down, which might affect data in transit, but data at rest usually wouldn't be harmed in any way; unless through some freak accident a server crash would render a drive unusable. But even then, servers are usually fully redundant, and have RAID systems in place that mirror the data, so kind of a dual redundancy. Plus actual backups on top of that; though with that amount of data they might have a priority system in place and not everything is fully backed up.