Error “Unexpected VHD Feature Mask ‘6’” on Hyper-V backup or replication job

The feature mask 6 error is Hyper-V specific. This KB covers that error and how to resolve it.

Feature mask 6 errors most often occur when a Hyper-V VM was a Physical to Virtual conversion using third party software. The issue is that some third party converter software creates the VHD/VHDX as a virtual hard disk with a Feature Mask of 6. This is not a feature mask recognized by Hyper-V and thus cause issues.

This is how the error presents itself:
 

To resolve this issue, convert the disk from Fixed to Dynamic or vice-versa (then convert back to what you had been using).

If you are you using  server 2012 host environments ONLY, you will need to convert the disk to a VHDX format (from the older VHD).

Both of these methods essentially create brand new VHD files, and move the data from the old VHD to the newly created one. This sets the proper VHD information and should resolve any related errors.