If I were to archive data on a hard drive, unplug it, and set it on a (not dusty, temperature-controlled) shelf somewhere, would that drive deteriorate much?
How does the data retention of an unplugged hard drive compare to tapes?
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Hard drives are unsuitable for anything other than short term archival storage. The problem is not one of data retention, it's the fact that stored drives have a bad track record for not spinning up.
Contrary to what Vilx- wrote, many of us know what happens to a drive that's been stored for 20 years - they won't spin up 90+% of the time. "Modern" drives are in fact worse than the drives of a decade or two back. CDs have been around more than long enough to know that the organic dye layer of an RW disc deteriorates surprisingly fast, especially when exposed to ultraviolet light (e.g. fluorescent lighting) and temperature fluctuations.
One thing that's often overlooked when comparing hard drives to tapes is the simple fact that backup tape technology is specifically designed to store data for extended periods. Hard drives are a temporary medium only.
Edit: The following was posted as an answer to a newer version of essentially the same question, so I'll merge it in here rather than have a second post
Hard drives are designed for relatively short term data storage. Quite apart from the mechanical issues there's the fact that magnetic media, and I mean ALL magnetic media, does deteriorate with age, even under absolutely ideal conditions. This doesn't require any external influences as magnetic particles are affected by others near by and in modern drives the density is nothing short of astonishing.
A drive in use has the magnetic strength regularly refreshed, either by actual writing to disk or by the drive's own low level system, which periodically reads and rewrites sectors. A drive in storage or even one that's simply powered down, receives none of that refreshing of the data.
As for how long a drive can be stored without becoming at least partially unreadable is an ongoing topic of debate and will certainly vary even across drives from the same batch, let alone different models or manufacturers. I personally would never rely on a device specifically designed for short term data storage for any significant length of time. Even tapes, which are designed for relatively long term data storage, should be refreshed ever few years.
With all that said, I don't know what your alternative options are. Gold layer CDs/DVDs are currently being claimed to have 20+ years of safe storage but I can also remember when the exact same claim was made for burnable CDs. Those early ones proved to have a safe life of less than two years.
The best answer re: archiving data is to move it to new media periodically and not to rely on any given type of media to "go the distance". If you want to see examples of "extreme" data archive maintenance read about CERN and their migration of data between tape "silos" (large robotic tape loaders with multiple drive and picker elements) of different tape technologies. (Those guys are generating a lot of data...)
Tapes last a long time but, as other posters pointed out, having a tape drive that is in mechanical working order, can still be attached to a computer, and has software support for the storage format on the tape is a challenge. You have to plan on moving data onto new medias as the old medias and their drives and software get old.
My strategy with my largest data-producing Customer (a court that records audio/video of trials and hearings) has been to write multiply redundant copies on LTO-3 WORM tapes of data, and to keep the data live on the SAN (but no longer subject to daily backup) so that it exists in at least 4 places at any time-- 3 of which are off-site. We've planned to move to LTO-4 when the current tape drives turn 5 years old (in 2 years). At that time we'll migrate all of the data on LTO-3 WORM tapes to LTO-4 WORM tapes. We'll also replace the SAN and migrate all the data there to the new SAN.
One of the rules of thumb is that the smaller the magnetic domain, the easier it is to flip it. One of the reasons that spaceflight hardware is so less powerful than what we use down here is because high density electronics are much more vulnerable to things like cosmic ray strikes and high radiation. Long-term storage has similar concerns, as the amount of entropy required to corrupt information goes down as your feature size goes down.
What is the degradation rate? Hard to say, we haven't had these things all that long.
Archival over the production lifetime of your HD interface? Hard-drives will probably last that long deactivated. Archival over 20+ years? I'd be really leery of that without regular refreshes and migrations to new hardware.
Only because only optimists seem to answer so far, I have to put my fly into the ointment. (Note that I have zero experience with storing HDDs).
Will your HDD demagnetize over time ? What is the strength of magnetic field near it's storage ? (earth magnetic field, speakers, motors, nearby power cables, transformators, wireless phone chargers).
Will it's motor get stuck ? Does it have electrolytic capacitor which will eventually dry-out ? Will you drop it to a concrete floor some day ? Will you connect it wrongly, or put it's electronics touching some metal screw and it will fry ? And so on...
I assume the answer to all those questions is no. Are you Ok with relying on this assumption ?
On you spare time, read this:
....I immediately grabbed all of my stored hard drives and started finding computers to connect them to so I could see if my precious data was still there. Ten percent of the external Firewire drives would not spin up. During one Steely Dan tour, we recorded all of the shows using a pair of Mackie 24-track hard disk recorders. We filled 70 20-Gigabyte drives. None of them will spin up now.
Since archiving digital data has been a subject of research for a while now, a number of papers and recommendations exist on this topic. Protecting digital assets for the long term by Joe Jurneke is a recent one trying a comparison between tape and HDDs for archival purposes. In summary, it makes the following points:
With all this in miind, it is near impossible to answer the "how long" question.
Current research indicates that given the right storage conditions, phase-change optical media and magnetic tape are both a decent choice for an archival duration in the magnitude of 10 years. In fact, quite a number of manufacturers offer "archival grade" versions of optical media like DVD+RW or Blu-Ray which are claimed to last 30 years or more.
A "true" archiving solution which would last a period in the magnitude of 100 years is yet to be found.
As there is a less easily replaced mechanical part to a drive, I would suggest that taps would last longer (if the tape real won't wind you can take the magnetic tape out and put it in another cassette more easily than you could move the platters of a drive if it stops spinning up or the heads refuse to move).
In either case there are three important things to do whether you use disks or tapes, assuming your data is important enough for the hassle (and presumably it is, or you would not be asking this question):
My guess is that it should be at least as good as tapes. Perhaps even better, because the magnetic disk itself is in a contained environment and magnetically shielded.
However, I'm pretty sure that nobody really knows what happens to a HDD after, say, 20 years of unuse. It's the same thing as with CD's and DVD's (the archival-grade ones) - they simply haven't been around that long. Well, OK, hard drives were there 20 years ago as well, but comparing them to hard drives of today is pointless.
For example, several years ago I had Seagate drives that didn't start if they stood around too long. The problem was in the composition of the bearing grease - which would slowly harden over time.
The solution in this time: switch on the computer and start gently knocking the drive from the side several times with the (wooden) handle of a screwdriver. If you had luck, the platter would start to spin again - if not, ...
Don't take anything for granted that has to do with hard drives.
Addendum: Another discussion on hard disk archive usage