Backup Support for External Storage (especially local storage)

How to use GitHub

• Please use the 👍 reaction to show that you are interested into the same feature.
• Please don't comment if you have no relevant information to add. It's just extra noise for everyone subscribed to this issue.
• Subscribe to receive notifications on status change and new comments.

Feature request

Which Nextcloud Version are you currently using: (see administration page)

Is your feature request related to a problem? Please describe.
NextCloud Supports backup only for its primary storage - Data DIR. setup gets complicated if you want multiple hard drives (HDD / SDD) for increasing your local Storage capacity.

NextCloud way is mounting via external Storage App - which also do have local Storage.

Problem is both Apps - this App & external Storage app - abandons backup support of added local storage

Describe the solution you'd like
A clear and concise description of what you want to happen.

Next Cloud Backup app - supporting user selecting list of configured external storage - to be included in backups.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

Start by reading how the Backup app handles the primary Data DIR and how the External Storage app exposes configured local storage. Identify the backup entry points and the list of external storages that would need selection. Done means selected external storage is included in backups without disrupting existing primary-storage backups.