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Do you guys know how much time needs to pass until the data stored on a hard drive starts to degrade?

To keep the data intact for long periods of time, I heard you need to periodically rewrite it on the hard drive, like every 5 years or so. Is it true?

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A similar question was asked some time ago on Serverfault: serverfault.com/questions/51851/… –  Linker3000 May 16 '11 at 22:00
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I remember reading a quote citing 35-40 years, I'll try to find it and post it for you. –  Breakthrough Jul 20 '11 at 10:36

8 Answers 8

up vote 35 down vote accepted

Overview

The longevity of the data stored on any drive depends on the conditions where it is stored and for how long. For hard drives, there are three main factors: magnetic field breakdown, environmental conditions, and mechanical failure.


Magnetic Field Breakdown

Most sources state that permanent magnets lose their magnetic field strength at a rate of 1% per year. Assuming this is valid, after ~69 years, we can assume that half of the sectors in a hard drive would be corrupted (since they all lost half of their strength by this time). Obviously, this is quite a long time, but this risk is easily mitigated - simply re-write the data to the drive. How frequently you need to do this depends on the following two issues (I also go over this in my conclusion).

To periodically refresh the data on the drive, simply transfer it to another location, and re-writing it back to the drive. That way, the magnetic domains in the physical disk surface will be renewed with their original strength (because you just re-wrote the files back to the disk). If you're concerned about filesystem corruption, you can also format the disk before transferring the data back.

You can also help to avoid this issue by archiving your data with recovery data and error correction when you put the data onto the drive. Many archive formats support the inclusion of data recovery algorithms, so even if you have a few corrupted sectors, you can still re-build the lost data.


Environmental Conditions

Some government organizations "sanitize" hard drives by exposing them to a very powerful magnetic field, effectively (and literally) removing the data from the hard drive by "resetting" all of the sectors. Do note that storing a hard drive in, or near the presence of magnetic fields (alternating or static) will severely impact the data stored on the drive.

Geomagnetic storms have been so powerful in some areas that they have actually corrupted hard disks in the past. If you worry about this issue, consider storing your drives in a basement or somewhere heavily insulated from the environment.


Mechanical Failure

Some people mention that they believe that the actual physical motor in the hard drive will fail long before the data on the disk platters degrades significantly. While this is an issue for a hard disk that has been sitting for a long time, if the disk is used once in a while (at least every 3-5 years), this should mitigate this problem.

That being said, I have personally heard of people booting up 10+ year old computers with no problems, the disks working perfectly. I don't believe this issue is much of a concern compared to the previous one, since you should refresh the data periodically regardless. That being said, do be aware that mechanical problems are the primary failure of hard drives (and recovering data off of platters is not a trivial task, especially in the future when it may be difficult to find legacy drives).


Conclusion

Compared to conventional long-term storage mediums (tapes, optical discs), the appeal of hard drives is quite apparent - they are small, easy to move around, have very good transfer rates, switch between computers with ease, and the data lasts for a fairly long time. But, like the two other storage mediums I mentioned, hard drives do not come without their own caveats. So long as you periodically "refresh" the data on the hard drive (and, in turn, ensure the mechanical aspects of the drive itself are still functioning), you should have no problems.

Depending on the priority of the data you've stored, you may want to refresh the hard disk more often. If it is essential data, I would recommend no less then 2 years at maximum. If you can withstand some chance of minor data loss (e.g. a few corrupted sectors here and there), go with 5 years. It doesn't take long to copy the data off the drive, and copy it back.

One thing not considered is the servo tracks and markings. These are written one time at the factory and never again (on modern disks). No amount of re-writes by the user or so-called low-level formatting freshens these. Once they fade, they fade!

It's different with the first stepper motor disks of the 80's. They don't have servo tracks and a low-level format writes ALL of the bits - fresh.

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When all sectors' magenetic strengths are 50% of their current strengths that means half the sectors are corrupted? –  Bavi_H Jul 21 '11 at 3:25
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If a magnet's strength after 1 year is 99% of its current strength, then it will take 69 years for it to be 50% of its current strength. Right? (.99^69 = about .5) –  Bavi_H Jul 21 '11 at 3:26
    
@Bavi_H yes, you are correct - sorry, I forgot about exponential decay. Answer updated, thank you for catching that. –  Breakthrough Jul 22 '11 at 19:36
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All modern hard drives rely on "servo" information on one of the platters to position the read/write heads. This is likely subject to the same magnetic decay, but there's no way to rewrite those servo tracks. –  ultrasawblade Jul 22 '11 at 19:51
    
if the drive is connected to the computer and any degradation is found on servo information then in the modern hard drives the servo information will be corrected and rewritten automatically larryjordan.biz/hard-disk-warning –  kaykay Mar 23 at 16:41

hard drives fail and are not good long term storage devices. but you do not need to rewrite anything on them...

if you want to keep data over long periods of time, get a tape recorder (computer tape recorders are used by large companies for their data)

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Do you mean a backup tape drive? –  uSlackr May 16 '11 at 21:40
    
I'm sure Dolores means a C64 Datasette. –  ultrasawblade Jan 10 '13 at 0:42
    
Magnetic tape is not a long storage device. Magnetic tape has the same issues hard drives do: moving parts and magnetic decay. Tape is a good backup medium, but those tapes are constantly being overwritten or refreshed over time. –  Keltari Jul 31 '13 at 13:22

I bought a very old Zenith 386 system with an RLL full-height hard drive, circa 1987 at a local thrift store for $5. Had 640KB of RAM and a 386 CPU.

Before you had IDE hard drives, you had MFM and RLL drives; these did not have the controller electronics on board but had ribbon cables that went to an ISA controller card. Very old drive.

Plugged it up, got it to boot (after figuring out the "BIOS drive type" - it was something like 70MB) just fine. Had MS-DOS 5.0 installed. Scandisk revealed 1 bad sector on the drive. Might have been caused due to movement, this is from the era where drives didn't lie about their bad sectors (and had a "defect table" sticker on it).

So I imagine if they are in climate controlled conditions and not subject to shock or vibration that they would last quite a long time.

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Probably more like 640*K*B :) –  nemo Jul 31 '13 at 12:00
    
haha, thanks for catching that. –  ultrasawblade Jul 31 '13 at 13:17

This will differ based on the type of HDD.

5 years sounds like a reasonable average, though.

For archival purposes, magnetic storage just isn't the best option. You need physical storage, such as optical disks. These have their own types of kryptonite, but are generally more suitable for archival storage than magnetic hard disks.

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The conventional wisdom is that you should revisit your data every five years to make sure that you can still read it. The general consensus is that the magnetic platters in the drive will start to degrade in 5 years of storage. The bigger issue is that storage technology changes. That means a format that works today will be unreadable 5-10 years from now.

The best option you really have is to have multiple copies on multiple formats and to check on the data at least once or twice a decade. That's really the only way to make sure that the data is both intact and on a format that can still be read.

If you can afford it, you could always pay someone else to manage it. Services like Carbonite can store lots of data for long periods of time. They also can provide disaster recovery services in case your loose your computer and local backups.

Hope this helps

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Note: Drive don't loose "charge" because they don't store things with charge. The mechanism is setting the local magnitization of the disk surface medium. That can be randomized by heat, shock, changing magnetic fields, cosmic rays (tiny chance), and enough time. –  dmckee May 17 '11 at 8:18
    
can you elaborate on this theory of "charge" in a hdd? I know for a fact such a thing does not exist so you should probably revisit the concept of magnetic storage: secure.wikimedia.org/wikipedia/en/wiki/Magnetic_storage –  Kyle May 17 '11 at 15:04
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Data is stored on a hard drive by using an electric charge to magnetize a small region on the drive platter. The resulting electromagnetic field represents the binary data. The high density of opposing magnetic fields interact in a way that slowly lowers the strength of this field. The fact that all magnets eventually degrade is why all magnetic media has a shelf life. –  Doltknuckle May 17 '11 at 15:49
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The use of the word "charge" was an attempt to explain a complex concept in a single word. Electricity and magnetism have a strong relation and a hard drive uses an electric charge to set the magnetic domains on the platter. Most people understand that batteries loose their charge over time just like magnetic field loose strength over time. "Charge" is an already understood term and is conceptually close to the behavior of a magnetic field. I admit that it might have not been the best choice of words so I rephrased that sentence to better match the reality of how magnets work. –  Doltknuckle May 17 '11 at 16:01
    
aw you should have @me. Good thing I decided to check back... downvote removed and a +1 for you :D –  Kyle May 17 '11 at 21:28

It depends entirely on the ambient electromagnetic noise, the density of the media, and the quality of the read/write head. More noise, more dense data, and lower quality read/write mechanisms will result in so-called "bit rot" setting in faster. It's also going to vary depending on the quality of the internal motor and bearings. Part of the problem of spinning up an old disk is the fact that they're mechanical, and mechanical things don't age well without maintenance.

On mag tape, I've always heard the limiting factor is the method they use to bind the magnetic particles to the tape backing. The shelf life in perfect conditions (no light, nitrogen environment, temperature controlled) for that is supposed to be about 25 years (which is a problem for hospitals, who are supposed to maintain data backups for the life of the patient). Tape continues to be used because it is cheap and very high density, not because it has a superior shelf life.

Recordable optical media lifetime is about 100 years in theory (note that the discs haven't been around that long to actually test!) assuming absolutely no light after burning. Once you introduce an environment with light to recordable media, however, the lifespan drops drastically. 10 years at the most is what I've been told. True pressed discs last indefinitely as long as the media is internal to the disc medium, but those are extremely difficult to produce and are not economical for one-off disc production.

This is what I remember from talking with a salesperson from Iron Mountain several years ago. We did not talk about magnetic disks, but I know they are not more reliable. Hard drives combine three elements: one magnetic, one mechanical, and one electrical. Any and all of these are prone to failure, and this is why hard drives are one of the most common devices to fail in an operational system. You'll also note that the most modern filesystems such as btrfs and ZFS do online "disk scrubbing" where the system will actively read and rewrite disk sectors to maintain the magnetic integrity of the disk.

I would say 10-15 years of life if the density is not ludicrously high and you sit the disk on a shelf and don't touch it.

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The data on the HD will last much more than two years, so there is a very good chance you can recover it.

The only problem might be that new hardware could be incompatible with old hardware, but two years isn't long at all.

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I'm about to used it as external HD... ::) –  winona Jul 20 '11 at 9:54
    
Should be fine. –  pavium Jul 20 '11 at 9:55
    
Last question, the hd has been partition twice. Is there any chance I can recover all the data stored there? –  winona Jul 20 '11 at 9:58
    
Difficult to say. If the data you need to recover was on the disk before the partition, it's gone now. Maybe (with more details) this would be a good subject for another Question. Asking more than one question at a time is frowned upon. –  pavium Jul 20 '11 at 10:06
    
"If the data you need to recover was on the disk before the partition, it's gone now." Incorrect. Utilities such as TestDisk can recover the partition structure, and if even that is impossible, something like PhotoRec can at least recover the files if a repeated erase hasn't been performed. –  Hello71 Jul 20 '11 at 14:25

Your question is simple but implies complicated answer.

Simply put, however, long term reliability and integrity of hard disk storage is unknown. Even if the magnetic area is still good and has your data intact, corrosion on the read/write heads can cause read or write errors.

That said, I'm fairly certain this shouldn't be a problem in your case, since the hard disk in question has only been stored and left untouched for two years.

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