Platters vs "heads"?

[ricno]

Back in the days you had to enter the exact number of heads, cylinders and sectors in BIOS to be able to use a new hard drive. From what I remember the number of heads were the number of "sides" of the internal disk plates. The number were often quite high and I think this was due to some limit in the numbers of this CHS way of describing the disks geometry.

Anyway, this days, is heads and platters the same thing? And how many "plates" is actually inside a modern SATA drive?

And, if there is more than one readable side, does this mean there are two read arms that is on both sides of the plates?

 

[sminlal]

There are normally two heads for each platter - one to access the top of the platter and one to access the bottom. However the manufacturers can build drives of intermediate capacities that don't use all the platter surfaces. For example, platters that store 250GB on each side can be used to build 250GB, 500GB, 750GB, 1TB disks, etc. just by varying the number of heads and platters.

I don't know this for sure, but I suspect they may build the intermediate-capacity drives in order to use platters that have a defect on one side - much the way the semiconductor manufacturers sell microprocessors in various speed ratings by testing them to see how fast they will run.

3.5" drives typically have from 1 to 3 platters.

 

[ricno]

Hello and thank you for your reply. So the "head" is really the read arm and there is one such arm (head) per side (platter) of the disks?

3.5" drives typically have from 1 to 3 platters.

Is it possible to have more than, say, 4 or 5 platters, due to size to have this internal arms?

Do you know if the different heads/arms can move individually or if they are all at the same position at the same time?

 

[fzabkar]

See this article:
http://hddscan.com/doc/HDD_from_inside.html

 

[sminlal]

> So the "head" is really the read arm and there is one such arm (head) per side (platter) of the disks?

The "head" is really at the end of the arm, but essentially yes.


> Is it possible to have more than, say, 4 or 5 platters, due to size to have this internal arms?

In the old days there were disks with a dozen or more platters - but these days most drives are built to a standard form factor. For a disk with 3-1/2" platters in a standard desktop-sized form factor, I believe that 3 platters is as many as the manufacturers could fit - although it's possible I'm wrong and there is a 4-platter drive out there somewhere.


> Do you know if the different heads/arms can move individually or if they are all at the same position at the same time?

Again in the old days there were drives with two sets of head actuators which could move independently of each other. But the limited space inside modern drive enclosures means that all of the head/arms are attached to one actuator motor and they all move in unison.

Another common misconception is that data can be written to or read from the disk through multiple heads concurrently - but in fact the tolerances of modern drives are so exacting that there's no way to keep all the heads lined up precisely with the tracks on all of the platters, so in fact only one head can be reading or writing at any given time.

 

[ricno]

Thanks for good answers.

For a disk with 3-1/2" platters in a standard desktop-sized form factor, I believe that 3 platters is as many as the manufacturers could fit

With these 3 platters, will they be located at two "discs" using two sides on one and only one side at the other?

 

[sminlal]

With these 3 platters, will they be located at two "discs" using two sides on one and only one side at the other?

No - a drive with three platters will normally have six access arms and heads - one for the top and bottom of each platter.

 

[cjl]

I believe that 5 platters is the most anyone uses (but it's fairly rare), with 4 platter being a more typical max. Most current 2TB drives are 4-platter with 500GB platters. This is all for desktop (3.5 inch) drives.

 

[ricno]

No - a drive with three platters will normally have six access arms and heads - one for the top and bottom of each platter.

Ah, then I did not really understood from the beginning - I thought the "platter" was one side of a "disc". But it would be that the "platter" is really one disc with two usable sides?

 

[cjl]

Correct.

 

[sminlal]

But it would be that the "platter" is really one disc with two usable sides?

Yes, as I indicated in my first reply. A "platter" is like a vinyl record - both sides can be used.

 

[ricno]

A "platter" is like a vinyl record - both sides can be used.

Great, thanks a lot.

 

[ricno]

Best answer selected by ricno.