I implemented recently VMware Data Recovery for a quick and dirty backup of a few virtual machines. After checking the backup, the most virtual machines ran properly but one virtual machine failed with the following error message: "Unable to access file <unspecified filename> since it is locked", combined with an error message inside the VDR log: "Trouble reading files, error -3942 (delete snapshot failed)".
After some investigation I discovered, that the backup snapshot hung on the VM (and was not visible in the Snapshot Manager), further one disk of that VM was still assigned to the VDR appliance! So I tried first to remove the snapshot with the known trick (first create a new one, afterwards perform a "Delete All"), but again I got that message above ("Unable to access file..."). Then it crossed my mind that I have to remove the disk first from the VDR appliance, but a simple reboot of the appliance didn't help, so I removed the disk from the properties of the VM, which worked! (take care in this step not to delete the disk file 
). After another reboot of the VDR the disk wasn't visible to the appliance anymore.
Now I investigated why the disk wasn't removed from the VDR appliance and so the removal of the snapshot failed. Then I remembered that the VDR appliance does a hot add of the virtual disk, which has to be backed up (as well as VCB in hot-add mode with a Helper-VM).
In the past I ran already into the issue, that snapshots of thin provisioned disks can not be created in datastores, which have not the proper block size. So I checked the disk size of the failed VM, it was 270GB - the blocksize of the datastore in which the VDR appliance resided in had only 1 MB (256GB max. filesize). So this seems to be the problem. A short search in the VMware communities confirmed my guess, so I moved the VDR appliance into another datastore which a larger blocksize.
Conclusion: Always deploy your VMware Data Recovery appliance oder VCB Helper-VM into a datastore with an adequate blocksize!
