The reader has a file in database/cache called media.xml. This file holds the metadata, collections and a listing of files on the device. If the file is not present Calibre cannot do anything with the device. You need to unplug the device from your computer and turn it off. Turn it back on. This should regenerate the media.xml file. If you are still getting the error about it not existing you need to make sure that the database/cache directory is present so it can be generated by the reader. According to that error message it was being detected as drive I.
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