Recently I had to do just that when a customer I recently on-boarded to our vSphere platform was having issues creating a VSS-aware backup. Even a quiesced snapshot command failed with an error.
This issue took quite some time to investigate. And I was lucky enough to have a support engineer from Microsoft guide me through the troubleshooting steps. As it turned out the backup solution was not the problem but it was rather a combination of virtual hardware, Windows and VMware tools. Maybe if you are unfortunate enough to encounter a similar problem you’ll find this useful.
Symptoms:
- EFI firmware
- Missing MSR partion(s)
- VMCI Sockets version earlier than 9.8.10.0 (VMware tools 10.2.1)
When you are using EFI for your VMs each disk needs to have a Microsoft Reserved Partition (MSR) partition. Which needs to be either 32 or 128MB depending on the size of the disk. The funny thing is that all the VMs that were impacted had disks that when extended – and rounded to the nearest non-decimal value in GB – had about 128MB of free space. What I think caused this, is when migrating the VMs using VMware Converter the MSR partition was not carried over. I haven’t found any documentation on this issue. But I did find a forum post on the VMTN from a VMware employee stating that VMware Converter 6.2 would support MSR’s. And sure enough, I used version 6.1.1. for my migrations.
Now the second part of the problem was an issue with VMCI Sockets (vsock). There is a known issue that should be fixed when using VMware tools version 10.1.10 as per the release notes. But I was running version 10.1.15 at the time and it still didn’t work. Upgrading to version 10.3.5 eventually fixed the problem.
1 Comment
Mike Anderson · August 9, 2026 at 2:38 pm
This is a really useful troubleshooting write-up, especially because the issue involved several layers rather than a single obvious cause. The connection between the missing MSR partitions, VMCI Sockets, VMware Tools, and VSS backup behavior is a great reminder that virtualization problems often require looking beyond the backup software itself. Practical case studies like this are extremely valuable for IT professionals dealing with similar environments.