Lost AVCHD Video Clips

On two occasions using my new Vixia HF 200 camcorder with 16 GB Class 6 Adata SDHC cards, scenes that I had recorded simply disappeared when trying to view them on playback. When I checked the card on the camcorder, the camcorder showed that significant memory was used! When I recorded additional clips on the same card, the new clips played back normally but the old ones are not visible even though the card info check on the camcorder shows that there has been increased storage. I believe that I may have closed the LCD panel while the card was still being written to, but I am not sure.
So I think the SDHC card retains ALL the clips but I cannot see them. How can I retrieve or see these lost clips. I do not want to format the card as this would delete the clips.
I use an Imac with OS X.
Thanks in advance.
Thibojb2

thibojb2