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It seems like you are trying to mount a second ADLS which is not exposed via private endpoint to the same Synapse Analytics workspace. However, the error message you received indicates that the public network interface on this workspace is not accessible. To connect to this workspace, you need to use the Private Endpoint from inside your virtual network or enable public network access for this workspace.
Based on the error message, it seems like you have not enabled public network access for this workspace. If you want to mount a second ADLS which is not exposed via private endpoint to the same Synapse Analytics workspace, you need to enable public network access for this workspace.
You can follow the steps mentioned in this document to enable public network access for your workspace: Azure Synapse Analytics IP firewall rules.
Once you have enabled public network access, you should be able to mount the second ADLS to your Synapse Analytics workspace.
As a best practice, the benefit of private endpoints is that it's no longer necessary to expose workspace endpoints to the public internet. The less exposure, the better.
For more details, refer to Azure Synapse Analytics security white paper: Network security and Understanding Azure Synapse Private Endpoints.
Please let me know if you have any further questions or if there is anything else I can help you with.
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