Here is a Final Cut Pro tutorial that will explain why media becomes disconnected and how to repair the problem.
Non-linear video editing is really about media management. Instead of cutting in a literal fashion, you really just reorganize and alter different digital files that have been created from the videos. What happens is you essentially import or capture footage into an editing project, which then puts them into a Capture Scratch or other storage location. The editing project then references those files when using them, but does not actually alter them in their base form. This is called non-destructive editing and it gives you the freedom to try a whole range of things without the possibility of ruining your original footage. One of the most common errors in video editing, with Final Cut Pro 7 or other video editing programs, is to have disconnected media. Since Final Cut Pro simply references that media in your project it has to be able to find the media each time that you open the project. It does this by having a path to that media, which is like an address of where it is stored. If your media is not in the location that it is looking in then your media will become disconnected. It is fairly easy to reconnect your media when this happens and to prevent it from occurring period.
Reconnecting Media in Final Cut Pro
To start reconnecting your media in Final Cut Pro you will have to first find the media in your storage drives. Oftentimes, your media will become disconnected because you moved its locations or it somehow got deleted. If it is still around you can locate it and then go back into your Final Cut Pro project. Right click the disconnected clips and choose to Locate the footage. When you get the browse prompt just go through, find the footage files, and select them. You can do this very quickly for all the files that may have been disconnected. If you actually have deleted the files you can often just recapture them without any problem. This will be true for the cut up clips in your Timeline as the timecode on those clips will often preserve its position. This way you can just recapture the video from tape or digital format, go through the reconnect stage, and in many cases you will find that it reconnects to this new video capture and maintains the video exactly as it was.
Set Scratch Disc
Organization is going to be the easiest way to avoid disconnected media in Final Cut Pro. Make sure to set your scratch disc every time you work and try to do it to the same area on a portable hard drive each time. When you back it up just transfer the entire project file over so that if you have to revert to the backup you can do so without any disconnection of the media. If you are bringing in stock footage or downloaded clips put them in their own specific folder and do not move them if possible.