I came across this article in July's Digital Camera Magazine. people may be aware but here it is -
"Lost shots for Lexar & Canon
Lexar CompactFlash cards and Canon DSLRs are in need of firmware upgrades after users report images disappearing from memory.
Leading memory card manufacturer Lexar and camera giant Canon have both issued warnings to DSLR owners using CompactFlash cards after customers reported the disappearance of images. Some owners of Canon's EOS 1 Ds Mark II and EOS 1D Mark 11 have found that a press of the Display button immediately after shooting in RAW+JPEG mode causes the cameras to lock up and lose all the images in the buffer. Meanwhile, Lexar Professional 80x-speed CompactFlash cards, in rare circumstances, have a tendency to lose images when used with the EOS IDs Mark 11, EOS 1D Mark 11, EOS 20D and EOS 350D.
Both companies are issuing firmware updates to overcome the problem but, in the meantime, Canon advises EOS IN Mark II and ID Mark 11 users to ensure the access lamp has gone out before activating Display. If the camera does lock up, removing and re-inserting the battery will get it going again. See
www.lexar.com/ support and
www.canon.co.uk/support if you're a Canon DSLR owner suffering bad memory."
Edit - EosD, noticed you have a 300 so it shouldn't apply.
regards