Hard drives are a lot like roads -- things move fast till about 80-85% of capacity, and then they start to slow down noticeably. Once you get into the 90%+ range, things can be crawling
I'd say that keeping your drive at about 65-75% full is pretty much the sweet spot. Buying a huge drive and only using, say, 10% of it at the most is a waste of your money, and having a drive that's 95% full will slow things down
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