While the drive path is visible in Windows, do not try to initialize or format these LUNs within the disk management snap-in, as this could corrupt or overwrite data stored on the VMFS LUNs. Further, note that the Veeam Backup & Replication v5 and above will automatically disable the automount feature of Windows. Automount allows for automatically mounting and assigning configuration to newly connected volumes. If you add a VMFS datastore to the Veeam Backup & Replication server, with automount enabled, the operating system may initialize and re-signature the volume. This would make it unrecognizable by the ESX(i) hosts.
Having these LUNs visible within disk management can ensure that all of the required LUNs are available to Veeam Backup & Replication, including viewing the target and LUN IDs as presented from the storage processor. Conversely, if all VMFS LUNs are not visible; there may be a zoning issue.
Direct SAN access processing mode allows Veeam Backup & Replication to communicate directly with the storage for the highest backup job performance. Further, if the backup target supports iSCSI or fibre channel; direct SAN access mode also enables a completely LAN-free backup implementation.