I suspected that might be the case. My iMac workaround was to rename the Calibre 5.1 App to calibre510 (which has also renamed the Launchpad icon) and re-install 4.23 as calibre.app with an alias on the desktop. Both versions work well and use the same library locations etc. I'm using 5.1 for all purposes other than wireless transfers to CC for which 4.23 still works OK, and will still have DeDRM available if I need it.
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