You can define a list of objects names to exclude from a backup operation. You can use standard regular expressions to match a set of objects. For example, if you want to skip all external objects, enter .*__x$.
You should exclude all external objects, as they are not supported. Backup operations that include external objects are expected to fail.
From the navigation pane, go to Protect > Salesforce.
The Salesforce page appears.
In the Name column, click the app.
The app details page appears.
On the Overview tab, in the Content section, click Manage.
The Content dialog box appears.
In the Exclude objects section, click Add.
The Add objects dialog box appears.
In the Object names box, using a blank space to separate objects, enter the objects to exclude.
For example, type Account Documents Attachment CustomObj__c.