Why are standard internal hard drives called 3 1/2"?

i_am_jim · Apr 5, 2017

I just came from a disputed discussion with someone over the width of internal hard drives.

She insisted they are 3 1/2" wide. I've measured them, so I insisted they are 4" wide. I have a fairly new Western Digital 1TB on the desk next to my computer. I just measured it. It's 4" wide. But, when I look on the web they seem to call them 3 1/2". Why are 4" wide drives called 3 1/2"???

JackNaylorPE · Apr 5, 2017

Put that in the same category as to why a 1 TB drive only has 0.93 TB of space on it.

But actually it kinda **approximates** the size of the platters or more accurately... HDs fit in the same width bay as 3.5" floppy disks. So a floppy drive that used 3.5" disks needed a slightly larger mechanism to read the disks. As HDs got manufactured to the same bay width, the designation stuck.

junkeymonkey · Apr 5, 2017

3.5-inch
4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³. This smaller form factor is similar to that used in an HDD by Rodime in 1983,[122] which was the same size as the "half height" 3½" FDD, i.e., 1.63 inches high. Today, the 1-inch high ("slimline" or "low-profile") version of this form factor is the most popular form used in most desktops. The format was standardized in terms of dimensions and positions of mounting holes as EIA/ECA-740, co-published as SFF-8301.[123]

https://en.wikipedia.org/wiki/Hard_disk_drive

USAFRet · Apr 5, 2017

If you take it apart, the platter(s) inside are 3.5" across.

junkeymonkey · Apr 5, 2017

all wrong its the mountinf holes as said above in my post from winki

''The format was standardized in terms of dimensions and positions of mounting holes as EIA/ECA-740, co-published as SFF-8301.[123] ''

i_am_jim · Apr 5, 2017

Thanks. Go figure, one would think they would use sizing for drive bays.

junkeymonkey · Apr 5, 2017

but he is correct about the platters being 3.5 ''' - its just one of them things as standard form factor designation

[some extra ]
https://superuser.com/questions/281613/what-are-the-differences-between-2-5-and-3-5-hard-disks

just l;ike a optical drive can be a 3.5'' form factor has no platters in it but its still can be a 3.5 '' form factor as well

or like this fan controller it a 3.5 form factor

https://www.amazon.com/HDE-Channel-Multi-Fan-Cooling-Controller/dp/B006UFMH3W

http://www.pcguide.com/ref/hdd/op/formIn35-c.html

USAFRet · Apr 5, 2017

junkeymonkey :

OK...the 'mount hole' spacing was standardized.
Has nothing to do with "3.5" "

An actual platter is 3.74"
The 3.5" nomenclature derives from being able to mount in the same space as a 3.5" floppy disk drive. In which the magnetic media was indeed 3.5" across.
As was a 5.25" floppy, and the 8", etc, etc.

Just like a DVD drive mounts in a 5.25" bay, whereas the actual DVD is 5" across.
(and as legend has it, the 5" DVD derives from a CD, which derives from an engineer at Sony who wanted it to be able to hold a specific amount of classical music)

JackNaylorPE · Apr 5, 2017

More here:

http://www.techrepublic.com/forums/discussions/why-do-they-call-them-35-inch-drives/

less than 1/4" from platter to case edge.

junkeymonkey · Apr 5, 2017

jack yyour wrong its how the form factor is standardized look at what I posted above has nothing to do with the platters

let me move that down to here for you

but he is correct about the platters being 3.5 ''' - its just one of them things as standard form factor designation

[some extra ]
https://superuser.com/questions/281613/what-are-the-dif...

just l;ike a optical drive can be a 3.5'' form factor has no platters in it but its still can be a 3.5 '' form factor as well

or like this fan controller it a 3.5 form factor

https://www.amazon.com/HDE-Channel-Multi-Fan-Cooling-Co...

http://www.pcguide.com/ref/hdd/op/formIn35-c.html

Paperdoc · Apr 6, 2017

USAFRet has the right answer - note especially his second paragraph.

An early computer storage medium was the 8" floppy disk, and the disk was 8" diameter. When desktop machines started to come out - early Apples, Radio Shack, Commodore PET's and then C-64's, etc., they all used floppy disks of 5¼" width, and the drives those fit into were a bit larger. IBM used those same floppy disk drive sizes in their first PC. When they introduced the PC-XT, their first model that included a HDD, they used HDD's that fit into that same slot size, so those drives became known eventually as "5¼" Form Factor Hard Drives". Later the IBM PS/2 models introduced a new floppy disk form, the 3½" diskette, with a drive that was a bit larger. No surprise - not too long after that came hard drives that fit into those same slot sizes, and they became known as "3½" Hard Drives". Now there are a couple of other smaller size units designed for laptops.

junkeymonkey · Apr 6, 2017

right after I posted it .

its a standard ATX spec form factor any device that's a 2.5 or 3.5 fits that speced form factor and mounting holes and bay that's as simple as m-atx and atx in a motherboard form factor

that's why a 2.5 device need a 3.5 adaptor to fit in a 3.5 and so on .

whats gets me is the experts did not even know that to start with . how many years building a computer ???

one more for fun-

''Form factors''
https://en.wikipedia.org/wiki/Drive_bay

JackNaylorPE · Apr 6, 2017

junkeymonkey :

Look what i posted above.... it contradicts your source. So does a ruler. Used to have a drive with a see thru cover, it measured 3.75".

look at USAFret and paperdoc

As the source said ... it's what goes in it, rather than what it is... kind a like when ya buy a diesel automobile ... it's not made out of diesel, but diesel goes in it.

junkeymonkey · Apr 6, 2017

your funny .. its the form factor has nothing to do with the platters in a hard drive how hard is that ??

what size are the platters in a fan controller that a 3.5 ??? or anything that speced as a 3,5
you need to check your info on why

''The format was standardized in terms of dimensions and positions of mounting holes as EIA/ECA-740, co-published as SFF-8301.[123]''

all cases all devices that are 3.5 fits the same and the same mounting holes not because the platters are 3.5

my gosh

silly if the platters were 2.5 in a 3,5 form factor then what ??? he did not ask what size the platters are he asked why ''Why are standard internal hard drives called 3 1/2"? ''

because that's the formfactor used any mounting for 3.5 is the same

USAFRet · Apr 6, 2017

Because it derives from the mounting dimensions of a 3.5" floppy drive.
In which the actual media was indeed 3.5" across (or close enough to 3.5" to be called 3.5")

The positioning of the mount holes is of no specific dimension that relates to "3 and a half inches"...just that particular spacing is what they standardized on, to maintain compatibility with a floppy drive mount.

If it related to the mount holes, we'd call it a 4" drive, or a 2.375" drive, or a 1.625" drive.
Or 4", because that's how wide the drive case is.

There...we are all correct.

JackNaylorPE · Apr 7, 2017

junkeymonkey :

I don't know ... why are you having so much trouble understanding it ?

I didn't say that it had anything to do with the platters other than the coincidental fact that the platters are near in size to 3.5 inches and some folks make this inference, and which I made every clear was 3.75".

I said it came from the form factor which was established based upon the size of the device manufactured to read 3.5" floppy dives.

So a floppy drive that used 3.5" disks needed a slightly larger mechanism to read the disks. As HDs got manufactured to the same bay width, the designation stuck.

Where exactly are you getting lost ?

4745454b · Apr 7, 2017

I wonder how many people know a 2"x4" doesn't measure 2" by 4"?

The platters are close enough to 3.5 that it's easier to say it. As for mounting holes, every standard is complete. Nothing is left to chance.

Paperdoc · Apr 8, 2017

Excellent point about "Two-by-Fours" that aren't. My understanding of the background there is that originally this common lumber piece was cut to pretty exactly 2" x 4" at the initial sawing stage, but of course that leaves a rough surface. Then they were all planed smooth, and that left them a little smaller in both dimensions. However, I suspect the industry since those days has shrunk them more than that at the first stage to save money, because their current dimensions cannot be blamed entirely on modest planing.

But of course, the point is that a label for something does NOT necessarily means what you THINK it might. History and habit may have changed things, but the handy name remains in common use.

Why are standard internal hard drives called 3 1/2"?

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