If the VM disk resides on a vVol, the Backup Proxy's VM configuration file must be on the same vVol.
The backup proxy VM must be located on a host that has access to the datastore containing the disks of the VM that is being backed up or replicated, or in the case of a restore operation, where the disks of the restored VM will be placed.
For example, when backing up or restoring to a datastore that is local only to a single host, hotadd will only function if the Backup Proxy VM is located on that same host so that it can attach the VMDK file to itself.
A single SCSI controller can have a maximum of 15 disks attached. To run multiple concurrent jobs with more than 15 disks, you need to add more SCSI controllers to your Veeam Proxy server that is responsible for hot adding the disks. (kb.vmware.com/kb/1037094)
HotAdd may fail if any disk was created with a newer hardware version than the VM being backed up. For example, if a disk was moved from a hardware version 8 VM to a hardware version 7 VM. To resolve, upgrade the hardware version of the VM.
In the case of a standalone host connection (no vCenter added to the console), you can only hot-add disks of VMs which are located on the same host as the proxy server. This may require additional proxy servers to be configured on each host in the environment.
HotAdd may fail if you are trying to back up the VM through the host added as a standalone server into Veeam Backup Infrastructure but actually being managed by vCenter. The following error message can be found in the source agent log:
"Reconfigure attempt failed for VM "" with Vmomi::MethodFault::Exception ("vim.fault.HostAccessRestrictedToManagementServer")"
If you plan to process VMs that store disks on an NFS 3.0 datastore, deploying a backup proxy on the same ESX(i) host where these VMs reside is advisable. (See: https://www.veeam.com/kb1681)
Virtual Appliance backup job fails after you’ve cloned or restored your VBR server. The reason for that is the incorrect UUID reference for the Veeam Backup VM, which can be found in the following registry key of the Veeam server. Please see http://www.veeam.com/kb1332
HotAdd fails if VM disks being backed up don't have unique ddb UUIDs across the vCenter, for example, if such VMs were deployed from a single template to different hosts. Please refer to the VMware KB article for details: http://kb.vmware.com/kb/2006865