An important distinction.
WHAT SYNCHRONISED STORAGE DOES
Keeps the same files everywhere.
WHAT THAT MEANS
A file deleted on one device is deleted everywhere. A file encrypted by ransomware is encrypted everywhere. A file corrupted is corrupted everywhere.
WHY THAT IS NOT A BACKUP
A backup is a separate copy at a point in time, which the live system cannot alter.
WHAT SOME SERVICES OFFER
Version history and a recovery period for deleted items.
Useful, and limited in duration and scope.
WHAT TO ESTABLISH
How far back versions go How long deleted items are recoverable Whether that covers a problem noticed weeks later
WHAT TO ARRANGE
A genuine backup of cloud data, separate from the service.
WHY
Account compromise, accidental deletion, and provider problems all happen.
WHAT TO BACK UP
Anything you could not recreate.
WHAT TO TEST
Restoring from it.
WHAT PEOPLE DISCOVER TOO LATE
That synchronisation faithfully propagated the problem.