I have been experiencing a problem with SD cards. One is a 1 GB card and the other is 2 GB. I use both with my digital camera and a digital picture frame. I ended up with just under 1/2 GB used in each card. The problem was discovered when I wanted to add additional photos to them. I received several error messages (including notice that disk was full). Yet xp and linux was reporting that they had unused space on both cards.
After several days of research, I find that the problem is not uncommon. I did not initially find any solutions to the issue.
Finally I found a utility to format SD cards. I copied all of the photos to my pc, held my breath and then used the utility to format the 2 GB card. The process ran and then reported that the format could not be completed. Figuring the card was toast I decided to format it with xp. It worked. The format completed. I then copied all of the photos to the card.
Here is what I think happened. First use for both of these cards was in the digital camera. In reading the manual, I find that the camera formats a new sd card. Apparently it formated the card with one 500 MB partition. XP reported 500 MB used and 1.5 GB free. Linux reported the same but also indicated there was 1 partition. That was the clue that finally led me to the answer.
Hope above info is helpful.