This page lists some notes and limitations of the backup process:

The user should define a mounted remote filesystem as the backup destination, see Configurable Parameters and Settings
HSM, iSCSI or RAID data is not backed-up
API Connect domains are not backed-up, except when in full device backup
Only persisted data is backed-up. Unsaved configuration changes will not be backed-up
Key materials are only backed up by Secure Backup
Administrative operations should not be performed during the backup execution. Doing so may cause failure or inconsistent results
Backup of IDG Docker containers is not supported (it may or may not work)

The backup can only be verified by restoring it to another device and validating its functionality

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