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I am not sure if you are using MARS agent backup on the above case, or you are using any other backup solution, but I am giving you the below answer considering you are using MARS agent to back up the 4 TB drive.
First, I wanted to inform you that there is no way we can combined the data for 2 different server names under one in Azure.
There are only 2 ways you can proceed:
- You can rename the new server as your old server which is "DG5-file-serv" and attached the disk to the server with same drive letter. Register the new server using the old passphrase so that it gets associated with the old server when you trigger the backup for the drive it would be a incremental backup and you would also be able to see old recovery points for the data source for restore purposes.
- You can stop the protection on old server with retain data and attached the drive to new server with different name and register that server to Azure. The first backup will be a full backup for 4 TB drive and reoccurring backup would be incremental backups. You can use the new server to restore data from the old server using the steps mentioned on below mentioned article. You can keep the old server data as per your retention and once that it completed you can delete the data for old server. Yes, this method will double your cost as your cloud storage usage would be around 8 TB.
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