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You can specify a particular mirror, if you like, but it's easier and more efficient to leave it the way you already have it. That way you'll get the fastest downloads. It also means that you rarely get instances of not being able to update while a mirror is itself in the process of updating. Choosing a nearby mirror doesn't always work - it depends on the loads on that server and what bandwidth they've allocated. I used to have a Debian mirror right here in Canberra. It was blindingly fast, when it worked, which was only about half the time. Now I just stick with deb.debian.org.