Reply to “NO CARD” Canon 450D

@Thomas Couto.

To answer your last question, Yes, I have an SD micro with adapter and it works in the XSi.

Are the contacts on your SD card dirty?

No idea if this will help at all but if you’re desperate for ideas:
Maybe try resetting your camera to factory defaults?

I wouldn’t be brave enough to try this when I know there is already a problem, but perhaps updating your firmware to the latest version?