Strategies for keeping library size in check

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So....I'm not sure I can offer much help because my strategy is to buy new HDDs....

But if you look at the Reports plugin, you could manually identify items that are unwatched for 6+ months or whatever your timeframe includes. Or see if you can write a bash script for a watch folder that auto-changes the date using the "date" function. Or if you utilize other automation tools, some of those can change dates as well.