Backups
Backup storage is not the same as file sync
File sync is useful for keeping the same files available in more than one place. It can also copy a bad change everywhere.
Deletion can sync too
If a file is deleted or overwritten, a sync service may repeat that change on every connected copy. A backup should give you a way back to an earlier state.
Keep backup credentials separate
A compromised server should not automatically have permission to erase every backup. Separate credentials and restricted access reduce that risk.
Keep another copy off the live server
RAID protects against some disk failures. It does not protect against accidental deletion, ransomware, filesystem damage, or loss of the entire server.
Test restores
A backup is only useful if it can be restored. Test a sample restore before depending on a new backup process.
Know how much history you need
Some workloads need yesterday's copy. Others need weeks or months of versions. Retention should match how quickly a bad change could be noticed.