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It seems the disk is by default the storage of the binary data of the images in Media asset (FileBlobProvider). The database contains only metadata, e.g. the references to the file system. So if somebody accidentally delete the blobs folder on the disk there is no way how to restore it and all images must be uploaded to the CMS once again.
I uploaded some images into Media. Then I create the urls for those images via UrlResolver and use this urls in tags. The appdata blobs folders are then created according to the web.config settings:
When I delete the blobs folder and all the subfolders then I do not get the image any more (http 500 response) and can see the exception in the log:
How can I recreate the blobs folder structure to get the site working again?