The error typically indicates a communication or access failure between the Veeam Backup & Replication proxy and the underlying VMware storage. The -flat.vmdk file is the actual data container for a virtual disk, and when Veeam cannot "download" (read) it, the backup or restore process halts. Common Causes
Failures often occur in HotAdd mode when a Veeam Proxy fails to correctly mount or unmount the source disk. veeam failed to download disk flat.vmdk
Underlying RAID errors or datastore corruption can make the -flat.vmdk unreadable. The error typically indicates a communication or access
This issue is rarely a single-point failure and often stems from infrastructure-level conflicts or configuration mismatches: Underlying RAID errors or datastore corruption can make
The service account used by Veeam may lack the necessary vCenter permissions to access the disk directly. backup fails after expanding source disk - Veeam R&D Forums