On set I copy cards to 2 drives as they fill up. You can use ShotPut Pro to copy to 2 drives at once and it verifies the copy. When the copy is done I do a quick scan of footage to make sure everything is there for sure and that my shots play. It takes a little time, but it's worth it. If you have a DIT or Data Wrangler/Manager on set they can do it for you while you continue shooting. Then for editing the footage is also copied to a RAID-5 array and the other 2 drives are kept as the original and backup, and additional media and project files are added to both as the project proceeds. During production and post there ends up always being 3 copies of all the footage, additional media, and project files (actually there's 4 copies of project files if you include the Time Machine backup). After the project is complete there's 2 copies kept on the separate hard drives and the RAID array is cleared for the next project. Doing it this way I have NEVER lost a single project, project file, clip of footage, media file, etc.
