Hard disk size - quoted & actual

[Grunff]

Hi all,

This is something I've often wondered about but never tried to find out
- why is it that when you buy a 400GB drive, hook it up to your
motherboard, you only get say 370GB? This is before it's been formatted
or anything.

I don't normally mind/notice the loss, but I've just built a RAID array,
and it was a bit of a shock that all I ended up with was 1.8TB instead
of 2.0TB.


TIA

 

[Peter]

This is something I've often wondered about but never tried to find out

- why is it that when you buy a 400GB drive, hook it up to your
motherboard, you only get say 370GB? This is before it's been formatted
or anything.

I don't normally mind/notice the loss, but I've just built a RAID array,
and it was a bit of a shock that all I ended up with was 1.8TB instead
of 2.0TB.

Capacity is the amount of data that the drive can store,
after formatting. Most disc drive companies, including Seagate,
calculate disc capacity based on the assumption that
1 megabyte = 1000 kilobytes and 1 gigabyte=1000 megabytes.

According to other sources:
1 megabyte = 1024 kilobytes and 1 gigabyte = 1024 megabytes.

Hence the difference.

 

[Grunff]

Capacity is the amount of data that the drive can store,
after formatting. Most disc drive companies, including Seagate,
calculate disc capacity based on the assumption that
1 megabyte = 1000 kilobytes and 1 gigabyte=1000 megabytes.

According to other sources:
1 megabyte = 1024 kilobytes and 1 gigabyte = 1024 megabytes.

Hence the difference.

But that doesn't quite account for the discrepancy. For example, my
drives are these:

<http://uk.insight.com/apps/productpresentation/index.php?product_id=HTGA03UOS>

They are quoted as 400GB drives, and yet my system sees them as 372GB
drives. The 1000/1024 difference isn't big enough - 372 * 1.024 = 381GB.


Any other explanations?

 

[Rod Speed]

Capacity is the amount of data that the drive can store,
after formatting. Most disc drive companies, including
Seagate, calculate disc capacity based on the assumption that
1 megabyte = 1000 kilobytes and 1 gigabyte=1000 megabytes.

It isnt an assumption, thats the standard SI unit.

And all hard drive manufacturers state the drive size that way.

According to other sources:

Who get it wrong. There is nothing intrinsically binary about
hard drive capacity. Thats only seen with ram and rom.

1 megabyte = 1024 kilobytes and 1 gigabyte = 1024 megabytes.

Hence the difference.

Indeed.

 

[Rod Speed]

Peter wrote

But that doesn't quite account for the discrepancy.

Yes, the rest is 'lost' to the directory structures etc after formatting.

For example, my drives are these:

They are quoted as 400GB drives, and yet my system sees them as 372GB drives.
The 1000/1024 difference isn't big enough - 372 * 1.024 = 381GB.

Any other explanations?

The directory structures etc, and in your case with RAID,
what gets used to keep the RAID info on the drives.

 

[J. Clarke]

But that doesn't quite account for the discrepancy. For example, my
drives are these:

They are quoted as 400GB drives, and yet my system sees them as 372GB
drives. The 1000/1024 difference isn't big enough - 372 * 1.024 = 381GB.


Any other explanations?

The drive manufacturer reports unformatted capacity--since they don't know
what file system you're going to be using they can't report anything else
and have it mean anything. When the drive is formatted a certain amount of
space is taken up by the structures the operating system uses to keep track
of which file is in which sector and provide services such as journalling.
The capacity that the OS reports is what's left after that has been done.

To take one well known example, the common 3.5" HD diskette has an
unformatted capacity of 2 meg, but formatted under DOS its capacity is
reduced to 1.44 meg.

 

[Eric Gisin]

It is 400GB / 1.024^3 = 370GiB.

 

[Folkert Rienstra]

and 1 kilobyte = 1000 bytes.

and 1 kilobyte = 1024 bytes.

But that doesn't quite account for the discrepancy.

Yes it does.

For example, my drives are these:

<http://uk.insight.com/apps/productpresentation/index.php?product_id=HTGA03UOS>

They are quoted as 400GB drives, and yet my system sees them as 372GB
drives. The 1000/1024 difference isn't big enough - 372 * 1.024 = 381GB.

372*1024B*1024B*1024B = 399.432GB = ~400GB
373GiB -> 400.5 GB, The 372 was likely rounded.

Any other explanations?

Nope.

 

[Folkert Rienstra]

Yes, the rest is 'lost' to the directory structures etc after formatting.
Clueless.



The directory structures etc, and in your case with RAID,
what gets used to keep the RAID info on the drives.

Utter nonsense.

 

[Grunff]

The drive manufacturer reports unformatted capacity--since they don't know
what file system you're going to be using they can't report anything else
and have it mean anything. When the drive is formatted a certain amount of
space is taken up by the structures the operating system uses to keep track
of which file is in which sector and provide services such as journalling.
The capacity that the OS reports is what's left after that has been done.

Sorry, perhaps I wasn't clear - this is the disk size reported by the
motherboard, before the disks have been formatted.

 

[Peter]

But that doesn't quite account for the discrepancy. For example, my
drives are these:

<http://uk.insight.com/apps/productpresentation/index.php?product_id=HTGA03U
OS>

They are quoted as 400GB drives, and yet my system sees them as 372GB
drives. The 1000/1024 difference isn't big enough - 372 * 1.024 = 381GB.


Any other explanations?

No, it is the same explanation:
kBytes to KBytes conversion takes 1.024
mBytes to MBytes conversion takes another 1.024
gBytes to GBytes conversion takes yet another 1.024

Now calculate:
372*1.024*1.024*1.024=399.432
assume you had not exactly 372 but 372.499999
372.499999*1.024*1.024*1.024=399.969
I think that is close enough to 400gB, ooops, they say 400GB ;-)

 

[Grunff]

Now calculate:
372*1.024*1.024*1.024=399.432
assume you had not exactly 372 but 372.499999
372.499999*1.024*1.024*1.024=399.969
I think that is close enough to 400gB, ooops, they say 400GB ;-)

Ahh...that makes sense. Many thanks.

 

[Kevin Buffardi]

Who get it wrong. There is nothing intrinsically binary about

hard drive capacity. Thats only seen with ram and rom.

Well, the basis of the measurement of a bit (Binary digIT) is of course
binary. A byte = 8 bits, and traditionally, these measurements are
portrayed in powers of two. So, it makes sense to represent bits or
bytes in powers of two. Of course, decimal arithmatic is more natural
for most people to compute (and it's probably easier to market products
with nice, round numbers), so there are tradeoffs.

//Kevin

 

[Rod Speed]

Well, the basis of the measurement of a bit (Binary digIT) is of course
binary.

Thats not what base 2 means in this situation.

A byte = 8 bits, and traditionally, these measurements are portrayed in powers
of two.

Thats just plain wrong. The 1.44MB floppy
is in fact a weird binary/decimal hybrid.

So, it makes sense to represent bits or bytes in powers of two.

Only when the total bytes are binary organised.
They aint with a hard drive or the cpu speed either.

Of course, decimal arithmatic is more natural for most people to compute

And is the SI standard.

(and it's probably easier to market products with nice, round numbers), so
there are tradeoffs.

Its much more complicated than that, as the 1.44MB floppy shows.

 

[Kevin Buffardi]

A byte = 8 bits, and traditionally, these measurements are portrayed in powers

Thats just plain wrong.

I suggest you read "The Principles of Computer Hardware" (Alan
Clements). It lays out the foundation of computer hardware in decimal
arithmatic. The acronym "bit" itself comes from Binary digIT, 0 or 1.
It's the origin of terminology, so yes, *traditionally*, bits are
represented in powers of two.

Note the word "traditionally." Not "always," but "traditionally."

 

[Kevin Buffardi]

It lays out the foundation of computer hardware in decimal

arithmatic.

Misspoken: "Binary arithmetic."

 

[Rod Speed]

Most obviously with hard drives. Also with telecoms.

I suggest you read "The Principles of Computer Hardware" (Alan Clements).

Dont need to, I know that is just plain wrong.

And I've been doing it since before Clements has too.

It lays out the foundation of computer hardware in decimal arithmatic.

Irrelevant to that particular pig ignorant claim.

The acronym "bit" itself comes from Binary digIT, 0 or 1.

I didnt even comment on that.

It's the origin of terminology, so yes, *traditionally*, bits are represented
in powers of two.

Note the word "traditionally." Not "always," but "traditionally."

Its still just plain wrong. Its only really been true of memory. And
that is because it does have an intrinsically binary ORGANISATION.

 

[Kevin Buffardi]

The acronym "bit" itself comes from Binary digIT, 0 or 1.

I didnt even comment on that.

Well that was my comment. Then you said it was wrong.

Its still just plain wrong. Its only really been true of memory. And
that is because it does have an intrinsically binary ORGANISATION.

I'm sorry if my first post was misleading, but I believe you are
misinterpreting it from my intentions. I never mentioned memory or
storage. I am talking about the origin of the terms bit and byte.
Those origins predate the use of the magnetic, non-volatile storage we
use today.

Both Peter and Folkert showed how binary arithmetic demonstrates the
difference between advertised size, and size represented in binary.

Your explanation of space taken by directory structures and such would
hold some credence if the disk was already formatted. However, Grunff
specified that the disk in question was unformatted and reported as
370GB by the motherboard (presumably in CMOS/BIOS).

//Kevin

 

[Kevin Buffardi]

I'm sorry if my first post was misleading, but I believe you are
misinterpreting it from my intentions.

Yes, upon reinspection I can see where I left too much of your quoted
text in, implying that I was referring specifically to storage capacity
when I was not. My apologies.

//Kevin

 

[Grunff]

372*1024B*1024B*1024B = 399.432GB = ~400GB
373GiB -> 400.5 GB, The 372 was likely rounded.

Yup, got it now, thanks.