I add books to Calibre in bulk so it's easier to auto convert when adding. That way I'm sure that the DRM is gone.
The tools didn't work consistently in 2013/2014. I didn't know this until I later went to open a Kindle freebie. Further examination revealed about 30% still had DRM attached. There was a timing issue with the DRM-removal process. I sent a bug report to Alf and they fixed the issue in an update a few months later. By that time Amazon updated K4PC and I had to redownload thousands of books one by one. I just recently finished.
To prevent that from reoccurring I auto convert when adding to Calibre. It's easier than opening them up individually.
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