Aside from having one master hard drive as the backup of my field recording archive (on a portable hard drive), I've only just begun to archive my 15 years worth of field recordings to DVD. This is a laborous process however, and I am looking to BluRayBlu-ray as more efficient method. As other answers have noted, sole reliance on portable hard drives isn't a good idea. When a drive containing a comprehensive archive of recordings failed a few years ago, I narrowly avoided losing the lot.