My system has a 750Gb drive that is about 60 percent full. I would use a second drive for my personal media, keeping it separate from the drive with the operating system and applications.
I have become a zealot about back-up and use a stand alone drive for copies of my media. I use an automatic process to back up the C drive but also periodically manually copy the media as well. This is not the most efficient back up system ever devised, but it makes me feel more secure.
I became a back-up believer the way most people do- painfully. A second drive in my system would allow for a safer and more labor effective back-up process. I'd still use the stand alone drive but wouldn't need to do the manual copying back-up to my back-up.
Our personal media is the most important data on our computers. Music can be replaced as can commercial video. My personal video and pictures are the most important items I own. And as zealous as I have been on back-up in recent years, a few weeks ago it became even more vital to me in a way that you Jason can appreciate: the birth 58 days ago of my first grandchild. The number of pictures on my computer has grown exponentially since then.
