Problem with Update 3a
Thanks to Anton GOSTEV for the information on the Veeam update 3a problem. Veeam is currently investigating a regression of Hot Add performance on VMs with a very large number of disks. Hot add times went to 20-30 min as opposed to normal 1-2 min. The temporary fix consists in putting the proxy in network mode. You will soon find in this article the definitive solution
What is network mode
The Network mode can be used with any infrastructure configuration. With this mode your data is retrieved by the ESXi host over LAN using the Network Block Device protocol (NBD).
Network mode is not a recommended data transport mode. Mainly due to a low transfer speed on the local network. Veeam Backup & Replication offers two alternative modes so as not to saturate the local network.
Network mode is the only applicable mode when the backup proxy role is assigned to a physical machine and the host uses local storage.
You can use this link for test hot add manually