If you are using VMWare Data Recovery appliance to do your replications, you may have come across the error “Failed to create snapshot for <vm_name>, error -3960 ( cannot quiesce virtual machine)”. This happens for many reasons. So if you have been looking for a resolution by Googling for “Failed to create snapshot for <vm_name>, error -3960 ( cannot quiesce virtual machine)”, you are most unlikely to have found the correct solution for your problem.
Best way to troubleshoot these steps are by actually going through your log file. You need to check the VMWare log file related to the virtual machine, you are having trouble with. You need to browse the Datastore related the machine to find this log file and it is called “vmware.log” Go through the log files to find out what exactly is the reason your backups/replications/snapshots are failing. In my case it was due to: “ToolsBackup: not enough empty nodes”.
This happens because each SCSI controller on ESXi can only control 15 disks and the quiesced snapshots in Windows 2008 and Windows 2008 R2 require one available slot per existing disk. So this issue will occur if the virtual machine has more than 7 disks attached to a single controller which was my case.
Easiest way to fix this is by adding a dummy Thin disk to the virtual machine from vSphere Client (No need to add this as a Hard Drive on the Guest OS) and attach the disk to a different SCSI node like SCSI (1:0). You should now be able to backup/replicate or create snapshots of the virtual machine.
Better alternative to fix the problem is by spreading the current disks on different SCSI controllers. But you will need to shutdown the virtual machine first and change the Virtual Device Node to new SCSI controllers from Edit Settings menu.
Good Luck. Hope this helps.