HotAdd Error: Failed to allocate SCSI target ID

A VMware VM Backup job utilizing Virtual Appliance (HotAdd) Transport Mode fails when attempting to attach disks to the proxy with the error:

Error: Agent: Failed to process method {HotAdd.AttachDisks}: Failed to open VDDK disk [[DATASTORE] vm_name/disk_file.vmdk] ( is read-only mode - [true] )
Logon attempt with parameters [VC/ESX: [server_name];Port: 443;Login: [domain\user];VMX Spec: [moref=vm-###];Snapshot mor: [snapshot-####];Transports: [hotadd];Read Only: [true]] failed because of the following errors: Failed to allocate SCSI target ID..

While there are mutliple reasons hotadd may fail, this article is specifically regarding a situation where there are insufficient SCSI nodes available on the VMware Backup Proxy virtual machine that is utilzing Virtual Appliance mode.
 

VMware Virtual Machines are limited to 15 disks per SCSI controller. If the VMware Backup Proxy has more tasks assigned to it than there are available SCSI nodes, the hotadd process will fail to attach the disks.

Consider the following options:

• Review the Proxy VM's configuration when no tasks are assigned to it and ensure that there are no stuck hotadded disks.
• Identify how many SCSI controllers the Proxy VM has and how many disks that VM has attached when no tasks are being processed. Subtract those disks from the possible SCSI node count and adjust the number of tasks assigned to the VMware Backup Proxy in Veeam Backup & Replication.
  
  For example, if a Proxy VM was built with a single SCSI controller, the maximum number of disks that can be attached is 15. If that VM has a single disk for its OS, that proxy should have no more than 14 tasks assigned.
  
  (# of SCSI Controllers * 15) - # of Disks Belonging to the Proxy = Maximum Hotaddable Disks
  
  (1 * 15) - 1 = 14
• Add more SCSI controllers to align with the number of tasks assigned to the VMware Backup Proxy.
• Reduce the number of tasks and instead build additional proxy VMs to share the load.
• Enable “Failover to network mode if primary transport modes fail or are unavailable” if it is disabled. This will allow the backup job to complete when/if Hotadd fails. The performance will be reduced, but the job will function.
   

Broadcom KB311427: Adding a SCSI controller to the virtual machine