Now that I've ripped about 1500 CD's I have around 100 GB of files. I add to these regularly and often edit or twiddle with tags so it's not very static.

I'm a backup fanatic - I back everything up to USB HD's. I keep a local backup in my home and I keep offsite backups - two 200G Maxtor One Touches that I swap between home and office, and two 250G WD Passposrts that I swap between home and safe-deposit box.

On the web I've seen all kinds of anecdotal reports that harddrives fail if left lying around for very long. The problem seems to be in the drive hardware - spindles getting stuck or heads freezing up, rather than data loss from failing magnetic domains. Allegedly the big Hollywood studios who keep their stuff on backup harddrives hire someone to spin them up every week.

But all the stories I've heard are rumours, FOAF stories, or individual personal experiences. I have no idea how many of them are true or, even if true, whether they are representative. If I had 100 WD Passports sitting on a shelf, after 6 months would I find that ONE had failed, 90 had failed or what?

Can anyone suggest any NON-anecdotal data about how long you can safely leave a HD sitting around on a shelf?

(N.B. that just backing up to a local HD that you keep running doesn't protect you if you have a fire or theft. Any good backup scheme must include an offsite component)

I'm curious as well... I have about ~15 drives that I keep at a relatives house in case of an emergency. Some are kind of older drives, all in all they range from 20 - 80 GB.. Would be nice to have an idea of their lifespan while not in use.

The problem with hard evidence like what you're looking for is that unlike media such as CDs and DVDs, HDDs are generally kept in use. As such, most of the hard data I'm aware of are for active disk drives. Some things you might want to keep in mind:

GOOD:
- HDDs are sealed environments. Unlike optical media, the write surfaces are not exposed to the elements
- Most electronics (of which HDDs are a subset) can be fired up a LONG time after purchase provided they were kept in good condition during that time. Cool, dry, shock-protected, etc.


BAD:
- HDDs don't take drops or any kind of rough treatment very well. One of the former and you could be done
- Interface tech can change rapidly. For example, most PATA/IDE hardware disappeared overnight once SATA became mainstream. Over the summer I had a problem with an IDE drive and could barely find anything that could read it

Bottom line, I'd say, is that if you wanna go the HDD route, just keep it running (in a server, in a PC, or something, even if you disconnect and take it with you for your "offsite" component ). HDDs generally give warning signs before they fail, so you'll know. Here's a paper on HDD life:

http://www.cs.cmu.edu/~bianca/fast07.pdf

Also, test your drives regularly for signs of mechanical failures. There are umpteen tools - both free and commercial if you want to do this. HDTune is a nice free app that I can recall off the top of my head.