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Why is the effective hard drive size lower than the actual size?

I have an HP Pavilion DM4 with a 750 GB hard drive. A few days ago, I installed Windows 8 on it, so I am now dual booting Windows 7 and Windows 8 (and I also have a 100 MB system reserved partition).

I noticed that I am only able to see 700 GB of hard disk space (169 for Windows 8 and 529 for Windows 7).

I booted of an Ubuntu 11.04 LiveCD and in the disk utility it said that my Windows 8 partition is 182 GB and my Windows 7 partition is 568 GB, correctly adding up to 750 GB.

I would like to reclaim the missing space in its respective partitions. Any advice on how to go about doing this?

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marked as duplicate by Oliver Salzburg Nov 27 '12 at 14:50

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Thanks for the prompt response everyone! That really helps me. I can see that this is the reason why I (incorrectly) thought I was "missing" space. However, one question.... Before I installed Windows 8, I saw the Windows 7 partition as 750GB as opposed to 698GB. Any idea why this is? Thanks again for your responses! –  bluedot951 Jun 27 '12 at 20:16
    
I've added an explanation of why you saw 750GB before installing Windows 8. Hope that helps clear things up. –  Synetech Jun 27 '12 at 20:23
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3 Answers

up vote 9 down vote accepted

The problem is that you are confusing binary and decimal prefixes.

You said it was 750GB right? That means 750,000,000,000B. Now convert that to GB:

750000000000/1024/1024/1024 = 698.5GB

That’s what you said you were seeing. You can repeat this for the individual partitions and you will get the same results.

Why does Linux give you different results? Because it is not dividing by 1,024 (the number of bytes in a kilobyte, number of kilobytes in a megabyte, etc.), but rather, dividing by 1,000.

This is a frequent point of confusion because hard-drive manufacturers always use SI prefixes (which go up in increments of 1,000, i.e., powers of 10) because it makes their drives sound larger, while software (especially Windows) uses binary prefixes (which go up in increments of 1,024, i.e., powers of 2).

So, you are not actually missing anything, nor is there anything to "get back". The "missing" space is simply due to a difference/disagreement in the interpretation of the term "gigabyte/GB".


As to why you saw it say 750GB before installing Windows 8, it depends on where you saw that. Did you see it say 750GB in My Computer? Linux? A system-information tool? Based on what you have said, I suspect it to be a Windows program as opposed to Windows explorer.

In this case, you may saw it say 750GB because the program you used is clever enough to be able to ignore the disk-space reported by Windows and instead look at the device id of the drive. Then it can look up the id in a table (or decode it, depending on the device), and use the official label and description for it.

As an example, let’s say that you have a Western Digital 750GB drive. The model number is encoded in the drive’s firmware, so Windows is able to see something like WD7500BPKT. Windows also sees that it has 750GB (decimal) of space available and stores that in binary as 698.5GB (or GiB if you like the baby-talk binary prefixes).

Simple programs like Explorer retrieve the free-space value and report that (698.5GB in this case).

A fancier program on the other hand does not ask Windows for the free-space value, but rather the device-id. It sees WD7500BPKT and either decodes it (in the case of WD drives, they encode the size in the model number), or else it looks it up in its database and says; hey, this is a Western Digital WD Scorpio Black 750 GB SATA 3 GB. Note the 750GB.

Alternately, a program may also just examine the total number of bytes and divide (repeatedly) by 1,000 instead of 1,024 to report the size as stated by the manufacturer. This is common with Windows ports of Unix tools.

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That really clears things up. Thanks a lot! –  bluedot951 Jun 27 '12 at 20:43
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I would check the disk size from within Windows, Run "diskmgmt.msc" from the Run Prompt (Windows key + R) and see what it shows as the size of those partitions, if its showing what you are seeing in Windows then thats probably all you are going to get, have a look and post what you can see so we can see whats going on!

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The reason you're seeing less hard drive space than you should be in Windows, is due thanks to the way that hard drives are marketed.

Let me explain: you see those drives going around, 1TB HDDs and stuff right? What you're actually seeing is the decimal notation of whatever storage unit. So when they say 1TB, they actually mean 10^12 bytes (or 1,000,000,000,000 bytes), not 2^40 bytes (1,099,511,627,776 bytes) as it should be. This discrepancy means that your drives are actually smaller than they should be.

The binary notation is usually prefixed with an i, so TB becomes TiB and GB becomes GiB and so on.

So how does windows calculate the drive size? Well, they use the binary notation, which is why it's smaller. Linux uses the Dec-10 notation, which is why you're seeing the drive size being 750GB. There is in fact a handy ratio you can use to calculate your true HDD size in binary notation, which is as follows:

decimal value / binary value = space discrepancy 

So for your Windows partition of 568 dec-10 gigabytes, you would do the following, which gives you a fraction which you can use for calculating binary hdd sizes-

(10^9/2^30) = 0.931322574615478515625

So...

568 * 0.9313 = 528.9784 Binary gigabytes

As you can see, it matches up to your Windows report of your hard drive. I hope that sheds some light on your plight.

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