A hard disk is made from one of more circular platters with magnetic material on one them. A read/write head floats on a thin air cushion over the platter. It can senses magnetic charge below it (reading). Or it can use a small electromagnet to change it (writing).
In the picture below the platter would be the gray disk. The green line would be the arm with the head at the end.
As you can see the head only covers a small part of the gray disk. Since the disk is rotating it will eventually cover all the spots in the light green ring.
If I move the head a bit to the side it will cover another circular track.
However both moving the head and rotating the disk takes time.
If I want to read some information which just happens to be below the head I can do quite quickly. If I want to read some information which just moved away from the head I will have to wait until that part comes around again. On average this takes half a rotation.
On a 7200RPM drive the platter rotates 7200 times per minute. That is 120 times per second. Or once per 0.008sec. That means when you want to read something new you will have to wait a 0.004 sec on average for the disk to rotate to the right place. (4ms).
It gets worse. Because the disk is not set up to read from just once small circle in the platter. it can move inward and outward to read many concentric circles. Moving a head also takes time.
If I summarize this it comes down to three small points:
- A disk can be busy reading data from disk
- A disk can be busy writing data to disk
- Or a disk can be busy doing other things, such as waiting for the head to arrive at the right position.
Using the pre-edit book analogy.
Say you are reading a story to a child. You have a book and you read a sentence or two. You then pronounce these so your child knows what the story is. You read more. You speak more.
But, eventually you reach the end of the page.
Your child will have to wait while you turn the page over before you can continue with the story.
That might seem to be something trivial. However with hard disk speeds your read speed is very high and turning a page (moving the RW head to another circle) takes a significant chunk of time.
If I let a disk just read from a single track it might read about 200 megabytes per second. This is about the same as a book reader who can rapidly read a page.
Now if I ask you to read the first word of every page things will slow down. But you will be very busy. Just not with reading but with turning pages.
Returning you your question, you wrote many times the read/write speed indicators are quite low, and the disk activity is high.
The read/write indicator is indicating how fast you are reading data from the disk. In MB/sec (or, bookish, in words/second).
The activity indicator is telling you how much time the disk is busy by either reading or writing data, or by waiting for the RW heads to get to the right place. If it indicates 100% then the disk is working as fast as it can. If you add additional tasks then you will have to wait longer until they complete because the disk simply has no time left to do more work in.
As why this is displayed as a percentage: I think it is because it is easy to read. 0% from being totally idle, 100% for working as hard as the disk can, and anything in between to indicate just how hard the drive is working.
Fragmentation (as asked in the comment).
Say I have a notebook with twenty numbered pages. I write a short story which takes up 5 pages. I then write three more stories of the same length. My notebook now looks like this.
page: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Content S1 S1 S1 S1 S1 S2 S2 S2 S2 S2 S3 S3 S3 S3 S3
I decide that story 2 is bad and I delete it with an erasor. The notebook now looks like this:
page: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Content S1 S1 S1 S1 S1 S3 S3 S3 S3 S3
I now write fourth and firth story. Both nice short ones, 3 pages long. I could either start writing at page 13 (after story 3). But that would waste free space and I would quickly have to buy a new notebook.
Or I could write my first story starting from the now empty page 3. If I do that my notebook now looks like this:
page: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Content S1 S1 S1 S1 S1 S4 S4 S4 S3 S3 S3 S3 S3
I then start to pencil story number 5. Starting with the first empty page. After writing two pages I run out of free space. At the bottom of page 10 I scribble 'continued at page 16' and write the last page of my story on that page.
page: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Content S1 S1 S1 S1 S1 S4 S4 S4 s5 s5 S3 S3 S3 S3 S3 s5
Now whenever someone reads my stories they can quickly read most of them. However story number 5 requires them to turn a few extra pages and thus costs a bit more time to read then similar 3 page stories. In other words, reading that is slower.
A similar thing happens with filesystems. The disk is the notebook.
If I open a book I have two facing pages. I can quickly read both of them. Similarly on a disk I can quickly read all the sectors on the same cylinder (e.g. the green ring in the picture). The sectors on the disk never get moved, they are like the page numbers in my notebook. However the story on it (files in a filesystem) can get scattered around the disk. This is fragmentation.
(And yes, this is a bit short as an explanation, but it was a side question)
