Hard Drives and Bad sectors

  Total Care Support 15:41 03 Feb 05
Locked

Hi All

I have just been in contact with a reader of the forums who has not yet posted with regard to an issue they have had. And it has pointed out a concern that people sometimes have and that that people that profess to be technical sometimes feed upon in order to get money out of unsuspecting customers.

The issue is bad sectors on a hard drive.

People see figures in the thousands or they just see the words bad sectors and they worry.

Hopefully we can help put some of the concerns regarding this issue to rest.

A average FAT 32 partition has sectors that are roughly between 32k and 14k in size (depending on size of drive and partition)

I will work with 14k for this example

A 30GB hard drive working with this figure would therefore have 2,142,857 sectors.

I have heard stories of people being sold new hard drives because they have 100 bad sectors by certain (to remain nameless) high street computer retailers, working with a 30GB hard drive this would be (and working with very loose maths) 0.005% bad sectors on a drive.

I have also recently been in a conversation with one of our customers where his company “IT “ guy advised that 48k of bad sectors was bad, this would only equate to only either 3 or 4 bad sectors, again the fact that the figure was so-much-k was making the issue more stressful than it needed to be.

At last count the industry standard laid down my hard-drive manufacturers was up to 10% bad sectors was classed as acceptable. (we generally work with a 5% margin) however even this figure can lead to many misconceptions.

What is not stated by Hard drive manufactures, and when it is its whispered quietly, is that you think that a sector is 1 however in most cases the manufactures build a redundancy into the hard drive sectors making each sector report as size 1 when in fact it is 1.3, this redundancy is not used until sectors start to appear as “bad” this redundancy is used to stop the lose of hard drive space.

If you have concerted to NTFT the sector size is even smaller and can even be reduced as low as 2k with some software, meaning more sectors on a hard drive and the ability for 100 “bad sectors” to be an even lower figure.

At present there, as far as I am aware no way to manufacture a hard drive with out the creation of some bad sectors (another reason for the redundancy) some drive manufacturers hide these when they seal their drives before shipping, however other companies do not, in many cases if you check the sectors on a brand new hard drive you will find some “bad sectors” this does not mean that the drive is faulty, far from it.

One final point is that the software that manages your computer will usually move data from bad sectors upon detection into the redundancy.

Hopefully this information helps

Best regards to all

Daniel Warren
Granville Technology
Forum.support@granvilletechnology.com

  Monument 16:40 03 Feb 05

"A average FAT 32 partition has sectors that are roughly between 32k and 14k in size (depending on size of drive and partition)"

This is not right.

A sector on a hard drive is 512 bytes in size.

The smallest addressable area is a cluster.

The cluster size on FAT32 will be 1, 8, 16,32 or 64 sectors.

"A 30GB hard drive working with this figure would therefore have 2,142,857 sectors."

Again an error a 30gb drive will have 30GB/512 = 58,593,750 sectors

  Stuartli 17:45 03 Feb 05

Hard drives that have developed recurring numbers of bad sectors are usually, in my experience, been on their way out.

  Maverick81 18:06 03 Feb 05

Lets excuse Mr Warren for a slight and unintentional error, I think its safe to assume he meant cluster size and not sector. We all know about the difference's in sizes with drives after formatting but as Daniel said bad clusters are a bit of a urban myth. Most drives made will have one or two bad clusters hard drive makers allow for this by making the drives a little larger than advertised bad sectors can occur without the user knowing because the drive simply remaps the information to the spare space.

As daniel said most drive makers allow for upto 10% of the drive to have bad sectors before declaring faulty. I think it is quite noble for Daniel to say 5% is the target figure before considering returns.

Lets face it if your missing 48k from the average 200GB hard drive nowadays it not like your missing much.

Just keep SMART enabled to keep a eye on your drive and make regular backups just to be safe.

The country is covered with people professing to be "IT experts" like anything else it life it pays to take more than one opinion onboard.

Well done to Daniel for at least trying to tackle this very touchy subject. What are the FE's thoughts ??

Cheers

Mav

  Stuartli 13:47 04 Feb 05

Two hard drives I have owned in recent years developed bad sectors, which became gradually worse.

Fortunately I had backed everything up regularly in each case before they eventually expired over a period of about five or six weeks.

  wee eddie 14:21 04 Feb 05

As Daniel says, a few bad sectors are more or less integral in the production of Hard Drives as are the occasional dead pixel on a TFT screen. Their presence below a certain level does not radically affect the operational capability of the product.

However, as Stuartli says, the development or increase in the number of bad sectors is symptomatic of a failing drive and anyone finding that there are more and more bad sectors on their HDD every time they defrag, would be well advised to overhaul their Back-up procedures and start researching the market for a replacement drive unit.

  CurlyWhirly 14:59 04 Feb 05

However, as Stuartli says, the development or increase in the number of bad sectors is symptomatic of a failing drive and anyone finding that there are more and more bad sectors on their HDD every time they defrag, would be well advised to overhaul their Back-up procedures and start researching the market for a replacement drive unit.


How would you KNOW that there are 'bad sectors' on your hard drive, as I was under the impression that the 'bad sectors' are kept HIDDEN from the operating system, and just remapped into the 'reserve space'?
This, I expect, is a marketing ploy to avoid alarming customers!
This is because if you ran a hard drive test and it came up with hundreds of 'bad sectors', this WOULDN'T exactly bestow confidence! LOL

  Dorsai 18:27 04 Feb 05

I think Bad sectors will be flagged as bad, and hidden, if there is some 'reserve space' to re-map them to.

But once the reserve space has all been used up, there is no where to re-map a new bad sector to, and so it can only be flagged as bad, but not hidden.

So then they start to show up. This is the point to worry. The drive maker expects a certian number of bad sectors to be present 'from new' and that over time a few more will develop. Hence the 'reserve space'. Once the reserve is gone, time to replace the HDD??

  CurlyWhirly 22:12 04 Feb 05

Thanks Dorsai for the explanation!
By the way, sorry to hijack the original thread (blush)

  Stuartli 15:09 05 Feb 05

I knew that the bad sectors were rising because the defrag utility revealed the information after the scan.

  Stuartli 15:18 05 Feb 05

The first drive was a 6.4GB and the second a 20GB - no doubt the effect of bad sectors would be greater than with one of today's three-figure examples (some of which cost less than the 6.4GB!)

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