Don’t the backup services themselves use HDDs to store your data? My point being, let us not rely on any ONE back up. Personally, I use several back ups...just in case.
Yes they use hard drives, however theirs are mirrored and backed up themselves. The chances of losing data in that environment are very small.
The problem I’ve had with keeping several backups is version control. Likewise, with this kind of service the file is backed up basically in real time. Manual backups are less frequent.