Thanks for proving my point that logically and physically there are two heads,
not one. That the heads can read and write at the same time and read after write
would be possible *IF* there were to be seperate Read and Write channels,
(which there aren't).
Ron Rough made the impression that is was impossible because there is only
one head. That point is false. It's because there is only one R/W channel.

"Marc de Vries" wrote in message
On Thu, 15 Jul 2004 00:38:50 +0200, "Folkert Rienstra" wrote:
snip
Is this a hardware issue ? ( Harddisk Read Head and Harddisk Write Head
can't work together like this and an extra seek is needed ? )

There are NOT two heads

Yes there are, you mighty clueless one.

Actually, the document you quote seems to prove you wrong.
A bit of sloppy writing in the document that confused you probably.

Did you consider that that may have confused *you* into thinking that
I am confused? Of course not!


"The head consists of a thin film inductive write element and an MR read element.

"The head consists..." Now, english is not my native language, but I
think "the head" clearly indicates ONE head. Otherwise they would
have said "the heads".

And obviously it is not the "head" that reads and writes but the read
and write elements.
And if you want to be picky (which obviously you are) then call it a
'slider', not a "head".

Yes, that part is indeed sloppy and can confuse what follows later.


But that single head has two elements: a write element and a read element.

Which are later referred to as read and write 'heads'. Thank you.


The read element is typically narrower than the write element to improve the
off-track performance. In practice, there is an offset between the center of
the read and write elements due to the longitudinal separation of the elements.
When used with a rotary actuator, the head is skewed with respect to the tracks
as the actuator moves across the disk. The result is a lateral offset between the
read and write head centerlines.

This statement is completely logical when you have one head with two elements.

Just as logical as a (one) slider has read and write heads.

It doesn't make sense when you would have two seperate heads.

That is what normal people will understand a read and a write element to be.
Logically they are 2 seperate heads rolled into one head element, the 'slider'.


Optimum performance is achieved by centering the read head over the data track
for read operations, and centering the write head over the data track for write
operations. This operation will cause the read head to be partially off-track
during a write operation. "

Here the confusion starts.

No it doesn't. It started earlier.

The document suddenly speaks about "read heads".

Because that is what normal people would call a read element.

But that doesn't make any sense when you compare it to the first part
of the document.

Right. Because it started off wrong there.
Which is understandable when most people will call a slider a 'head'.

The confusion starts when the technical terms and layman terms are used
intertwixed and sliders and elements both get simplified into "heads".

But if you replace the words "read head" and "write head" with
"read element" and "write element" it makes perfect sense again.

Source: IBM/HGST http://www.hgst.com/hdd/ipl/oem/tech/noid.htm

Looking at that document it becomes crystal clear that there is only 1 head

No, there is only one 'slider' with the read and write 'head' on it.

with a write and a read element, but that the author sometimes
uses "read head" where it should really be "read element".

Or used "head" when he should have used 'slider'.


but just a single head per disk surface. That head both reads and writes
No, it doesn't.

Yes it does. It's clearly shown in that info from IBM/HGST.

The slider does not read or write, the head(element)s do.


but can do only one or the other at a given instant.
There is a finite switch time between read and write mode.

Because of the 2 heads, one aligned behind the other.

One head, with 2 elements.

One slider with 2 head(element)s.

One behind the other.
As you can clearly see in figure 4 on the webpage you have pointed at yourself.

Wow, you can read? But can you understand?
So yes, why in the first place do you you think I pointed to that page?
To embarresh myself? Get a clue.



Marc