Good explanations, thanks. It may be worthwhile to note that if there is some process that is accessing the data on either card and you open the back panel or external access door for that card, you will get a warning "Memory card in use. Removing card may damage data..." I find that it sometimes takes a few seconds for the warning to appear (especially on my 4GB "external" card. (And it's not always obvious that a process is running - there are background processes such as the meta crawler that access the cards without the user's knowledge. At least that's my experience.)