# What does MB/s stand for in HDD access speed?

In relation to the hard drive access speed, what does MB/s stand for -- is it Mega Bit per second or Mega Bytes per second?

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The only real answer to that question is: You will never know. – Oliver Salzburg Feb 13 '12 at 11:36
Haha. Well, the real difference is in 8 times :) – ahmd1 Feb 13 '12 at 11:37
@ahmd1; the difference could also come from the Mega part ;) Is it 10^6 or 2^20 ? :) But the question wasn't phrased this way... – Karolos Feb 13 '12 at 11:47

Standard convention is that capital B is bytes, lower b is bits. I don't know if this is always adhered to, but it seems to be in my experience at least.

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And actually it should really be MiB, meaning bytes refers to the binary power of 2^10 (a bit over a million) rather than 10^6 (exactly (arithmetic) million). – deed02392 Feb 13 '12 at 14:28

If you are referring to the figures in the Windows copy dialog, then it means 1 megabyte (1048576 bytes).

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I took it from the Windows 7 window when one copies a file. – ahmd1 Feb 13 '12 at 17:50

Usually, B stands for bytes and b for bits; Mi for Mega (2^10 * 2^10) and M for million (10^6), but also, in computing, as meaning the same as Mi. This is often agree for transfer speeds.

However, when talking about raw disk capacity, disk vendors reportedly say 10 GB to mean 10 billion bytes, which is different from 10 GiB = (2^10)^3 bytes = 1073741824 bytes ~ 1.07 billion bytes. But some software use this convention too (however that's not too common). For instance, since Snow Leopard, Apple has moved to follow the hard disk vendors on this approach.

Edit following the comment of @Ambroz Bizjak:

Indeed, in 1998 conventions have been adopted that define Mi 2^20 and M for million (10^6). However, many popular computing applications still use the old prefixes, e.g., from the find man page:

``````-size n[cwbkMG]
File uses n units of space. The following suffixes can be used:
'b'    for 512-byte blocks (this is the default if no suffix is used)
'c'    for bytes
'w'    for two-byte words
'k'    for Kilobytes (units of 1024 bytes)
'M'    for Megabytes (units of 1048576 bytes)
'G'    for Gigabytes (units of 1073741824 bytes)
``````

So everything depends on the context.

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Your understanding of M and Mi is incorrect. Mi (mega binary) specifically stands for 1024^2 = 2^10 * 2^10, while M (mega) is ambiguous; it's usually the same as Mi, except in for instance hard drive capacities, where it stands for 10^6. en.wikipedia.org/wiki/Binary_prefix – Ambroz Bizjak Feb 13 '12 at 11:53
@AmbrozBizjak: Thanks for pointing this out. I didn't mean to use that convention. I just want to distinguish the two. The point is that computer people have used M in the binary sense in the same way as all the rest has been using it in the decimal sense. The IEC conventions you mention are only dated from 1998, which is relatively recent to break old habits. But I will edit my post to conform to that. – Karolos Feb 13 '12 at 12:04