With all that was siad above, you can encounter problems with your cards (sandisk for example). I had a problem where it said I only had 3 megs free when I should have had 110 free. I downloaded StorageTools from SoftWinter (
http://www.softwinter.com). It allos you to do a scandisk and fix problems, and it worked great. It also allows you to format using fat32 which is nice if you have lots of small files on your cards.
McCade