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The cheapest option would be to create a multi-part archive of the content of its hard disk, e.g. into 4.38GB files and write them to a series of DVDs. The catch with the archive method is that you’ll need to extract all the content to be able to edit footage later on. Another drawback is that a defective DVD could prevent any content of this and the following DVDs of the archive from being read, quite a common issue back in the days of floppy disks, when one bad disk in a spanned archive could spoil most the archive.
Another option which takes a little more time is to split any files longer than 4.38GB using your video editing software such that they will all fit on DVDs. In this method, I would suggest doing it by footage type, e.g. DVDs 1 & 2 for Christmas, DVD 3 for (some name)'s Birthday, etc.
With how cheap hard disks have become, my preferred method is to use two hard disks (1 internal + 1 external or 2 external) and this is how I store my digital photos and video footage. For example, a 1TB disk would hold a lot of HD footage. As it’s risky storing everything on one hard disk, by using two, you can use one as the main drive for your recordings and the second to periodically copy any recently added footage to, to avoid the risk of either hard disk ever giving a problem.