There is a simple program for Windows called h2test which is designed to test flash media, it does this by writing a series of large file to the flash media then running a checksum on them to make sure it was written and reads correctly, it may be worth trying if you are not sure your sd card is working properly. It also runs on Linux using Wine.

And to correct an earlier post: There are open source Linux drivers for the filesystem used by SDXC cards, so while Maemo doesn't currently support it, it is at least a possibility.