DCR DVD-108 Why is this happening....

I really need help with recovering footage from mini dvd's. I am using a Sony DCR - DVD108 camera, and my computer is an imac 10.5.2. Both are new so I am just learning. I have to finalize the disc's in my camera then using a USB transfer them to my computer (obviously) Every other disc that I finalize when I go to put it on the computer the computer says it is reading a blank disc. When I go and try to view the footage in my video camera it also says the disc is blank. I know that they weren't because occasionally I will review the footage I had just taken before I got home so I know it was on there. I can't figure out what is happening and is there anyway to retrieve the footage that was on the disc. If I am not in the right place can you point me in the right direction? Thanks so much!!

Could you clarify a few points?

You could review the video clips in the camcorder before finalizing, but could not review them in the camcorder after finalizing?

When you say "USB transfer", I think of having the disc in the camcorder and transfering the video to the PC using Picture Package. You've got a Mac though, right? I don't think Picture Package works on them. Are you trying to see the finalized miniDVD in the camcorder as an attached drive on the Mac?

Do you have a DVD reader on your Mac? If so, have you put the finalized disc in it?