Question: What does a rare-earth magnet do to a USB stick?
Answer: (Having accidentally carried out this experiment by having both in my desk drawer at the same time): It sticks to it.
I was rather relieved to discover that the data on my USB stick seems to be perfectly intact, despite the casing of the stick being stuck to the magnet for a while.
Actually, it shouldn't have surprised me. The mechanism for flash memory (see here, for example – it's a bit techy but you might get the idea) needs very substantial electric fields (of order ten volts over a tiny length scale) to change anything. Once electrons have been put on the floating-gate, they won't shift for a long time – unless the right electric fields are applied to bring them back again. While waving a magnet at it will certainly create electric fields (that's Faraday's law), it isn't going to come close to changing anything in a flash memory.