>We are designing a device that includes some relatively strong magnets. The device is about the size of a cigaret pack and is connected to a laptop using USB. I am afraid that at the end of the day when I throw the device in my computer bag along with my laptop and my wallet that the magnet will de-magnetise my debit cards and erase my harddrive (or even flip a couple of bits).

Some devices use exotic rare-earth permanent magnets to purposely erase (degauss) hard drives and other media ( http://www.fujitsu.com/us/services/computing/storage/degaussers/ ). If your magnets are in that category it's probably not a good idea to get either hard drives or mag stripe cards anywhere near them.