If the Veeam Proxy being used to process the source VM is on a VMFS 3 datastore it must be formatted with proper block size to be able to mount the largest virtual disk of the hot added VMs:
1MB block size – 256GB maximum file size
2MB block size – 512GB maximum file size
4MB block size – 1024GB maximum file size
8MB block size – 2048GB maximum file size
The Proxy being used to back up the VM must be a Virtual Machine on a host that is able to access the datastore where the source VM’s disks are stored
For vSphere 5.1 the maximum supported VMDK size is 1.98 TB
For vSphere 5.5 and older the maximum supported VMDK size is 62 TB
HotAdd is a SCSI feature and does not work for IDE disks
VMware Tools must be installed and up-to-date on the Veeam server and any Veeam proxies
Datastore needs sufficient space for a VM snapshot
Backup and restore of SATA disks in the Virtual appliance mode is supported if you use VMware vSphere 6.0 and later
If the VM disk resides on a Vvol the Hot Add Proxy VM's configuration files must also be on the same Vvol.
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.
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)
In case of 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 the NFS datastore, you must deploy the backup proxy on the same ESX(i) host where these VMs reside.
Virtual Appliance backup job fails after you’ve cloned or restored your VBR server. The reason for that is incorrect UUID reference for 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