Effective while its on. The adhesive is very strong and the cover will not come off, but due to the fact that it is a magnet the slider cover will come off on anything it can stick to. I put mine on my phone and it'd have to find it every now and then in my purse, or stuck to my keys, but I'm sure it will do much better on a laptop since those typically are closed and when not in use, and probably don't come into much contact with other metals.
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