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Unfortunately i missed your post last month, because i can answer your questions. I had the same error, but it is a really easy one, because your image is attached to the WDS. In the WDS the image is stored in two files, one is a Wim-file, the other is a rwm file. WSIM can´t use the rwm-file, because it is only dealing with the wim-file. Therefore you get the error message. Solution: You have to export the image from the WDS in a complete wimf-file, apply the changes you want to make, and bring it back to the WDS.