Failed to allocate SCSI target ID... during HotAdd

Challenge

Whenever you use the Virtual Appliance (HotAdd) Transport Mode (Failover to network mode if primary transport modes fail or are unavailable is disabled) to back up the virtual machines, you may encounter an error stating that Veeam has failed to attach disks.
 

Cause

This is most likely caused by the lack of available SCSI ports on the proxy (VDDK logging shows "Failed to allocate SCSI target ID"). You can find the following entries in the agent logs (Agent.%VMname%.Hotadd.Attacher.log):
 
 < 388> vdl| 2017-01-08T07:03:14.974+01:00 error -[00388] [Originator@6876 sub=transport] Mounting VM vm-170 using transport hotadd failed : Failed to allocate SCSI target ID..\n
 < 388> vdl| 2017-01-08T07:03:14.974+01:00 error -[00388] [Originator@6876 sub=transport] Cannot use mode hotadd to access [TEST1234-LUN20-Mixed] VM02/VM02_1.vmdk: Cannot mount using this method. (Mounting VM vm-170 using transport hotadd failed : Failed to allocate SCSI target ID..)\n
[08.01.2017 07:03:14] < 388> vdl| VixDiskLib: Unable to locate appropriate transport mode to open disk. Error 13 (You do not have access rights to this file) (Mounting VM vm-170 using transport hotadd failed : Failed to allocate SCSI target ID..) at 4570.\n
 

Solution

Try adding more SCSI controllers to their proxy, or creating more proxies, or enable “Failover to network mode if primary transport modes fail or are unavailable” if it is disabled.
 

More Information

Adding a SCSI controller to the virtual machine (1037094)