When an command to take an image or take science data is given, it gets a priority tag ( from 0-99 I believe ) with operation critical imagery and data set as top priority, and just postcard type stuff set at a low priority.

Unless commanded to do something specifically different - during each UHF comms pass, they start dumping out the data products from the highest priority first. Essentially, managing its own filing cabinet with the important stuff at the front.

Now they're back out on the plains at Meridiani - they can get some EXCELLENT horizon to horiozon UHF passes - which means they're getting more data off opportunity. Given that its had a couple of sols of little science activity, and just driving - it's been an opportunity to dump the data products on board all the way down to really really low priority stuff. What this means is that the flash memory is now very nearly empty and ready to take on a huge quantity of data during the next big stop on the way to Victoria ( a full pan, Mini TES pan work etc ) - and not worry about having to do it piece by piece over half a dozen sols because there's no room or bandwidth to send it back to Earth. They can just dump it all in flash - check it's ok with a few thumbnails, and start on the next long drive, dumping the data back during periods of low science.

Doug