Azure Blob Storage is the correct choice for storing backup copies and long term archives of large unstructured files.

Azure Blob Storage is an object storage service that scales to store very large amounts of unstructured data. It supports access tiers for hot, cool, and archive data so you can optimize cost for long term retention, and it provides lifecycle management, snapshots, immutability features, redundancy options, and server side encryption which are useful for backup and archival scenarios.

Azure Disk Storage is not ideal because it provides block storage for virtual machine disks and is intended for low latency OS and data disks rather than large scale object archives or long term backup stores.

Azure Cool Storage Tier is not a separate storage offering. It is an access tier within Blob Storage for infrequently accessed data, so the full service is still Azure Blob Storage rather than a standalone product called Cool Storage Tier.

Azure Files Storage offers managed SMB and NFS file shares and is aimed at lift and shift file workloads and shared file systems. It is not optimized for massive object archives and does not provide the same object tiering and lifecycle features as Blob Storage.

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