The main thing for me is that I have both my Calibre library (backed up various ways) and a separate folder containing deDRM-ed files (also backed up).
I do this because my Calibre library is on Dropbox and I access it from different computers using different OSes. And over in the Calibre forum they'll tell you not to do that, that it can cause problems. And it has - though not that often - and so I like to know I could re-create my Calibre library from scratch if necessary. But mostly I've just had to occasionally re-add an individual book file (maybe 2-3 times in the last 5-6 years?)
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