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MikeyTsi - if not already done try the following,
- install the portable version of calibre and do a simple test on its default library,
- revert back to 2.55, you'll need to uninstall 2.66 via the CP, but you wont lose libraries or config data, old versions are here ==>>Previous calibre releases
- create a dedicated folder for calibre's temporary files - e.g. C:\!CalTemp, and point the CALIBRE_TEMP_DIR environment variable at that folder, see Environment variables
BR