Item cannot be shared while still referencing media on camera

Haven't had this problem for months. When downloading the files from SD card today, it hung up at 99% complete for about 5 minutes, then the download ended. When I started looking at the video in the browser, the end of the video started to disappear. I sent the video to Trash, restarted FCP and attempted to download the video again. This time FCP crashed. I reopened FCP and attempted the download again. This time the download appeared to work. I was able to download the video to the timeline and work with it. Upon completing the timeline edit, I restarted FCP to proceed with copying to DVD. That is when I got the error that I can't share and when I noticed the Quicktime notation was missing and the camera icon was in the upper left corner in the browser video area. QUESTIONS: why did it happen this time? Do I have to start over time line editing again from the beginning? How can the problem be prevented in the future?