Others have suggested that you use smaller, less expensive sticks when doing the swap so that if the stick is somehow ruined, your wallet will not take a big hit. I think this is sound advice untill the problem is more fully understood.
I had a similar problem with my 1GB Sandisk stick and neither my PC or PSP would recognize the stick. This happened after my PC was writing to the stick and my PC locked up in the middle of writing. After that I could not format it or anything. The PC would start to read the stick and then start over attempting to read it and this would go on in an endless loop. I had to send back to Sandisk for replacement.