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General question about HDD bad sectors

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bobba84

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Jan 6, 2014
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Melbourne, Australia
Hi guys,

While formatting the Miniscribe 20MB drive in an XT clone I picked up, it started hitting bad sectors about halfway through. Now I know what bad sectors are, and how the system deals with them. But what I don't know is, why did they get the stepper motor to slowly crawl back to track 0 each time it hits one? It seems like a monumental waste of time - with more than a couple bad sectors it changed a 5 minute format into a 30 minute one. I've seen seagate drives do this too - and I already figure it goes back to track 0 so it can try jumping to the track with the bad sector again - I just can't figure out why it would jump to every track in between on it's way back? Can anyone shed some light on this behaviour?

Relevant YouTube video below:

http://www.youtube.com/watch?v=HMJl8Cw2OxM
 
bobba84 said:
Hi guys,

While formatting the Miniscribe 20MB drive in an XT clone I picked up, it started hitting bad sectors about halfway through. Now I know what bad sectors are, and how the system deals with them. But what I don't know is, why did they get the stepper motor to slowly crawl back to track 0 each time it hits one? It seems like a monumental waste of time - with more than a couple bad sectors it changed a 5 minute format into a 30 minute one. I've seen seagate drives do this too - and I already figure it goes back to track 0 so it can try jumping to the track with the bad sector again - I just can't figure out why it would jump to every track in between on it's way back? Can anyone shed some light on this behaviour?

Relevant YouTube video below:

http://www.youtube.com/watch?v=HMJl8Cw2OxM
Click to expand...

You might try running CHKDSK and then Steve Gibson's SpinRite if all else fails.
 
Agent Orange said:
You might try running CHKDSK and then Steve Gibson's SpinRite if all else fails.
Click to expand...

Thanks for the reply - I forgot about SpinRite! I'll give it a go!

But... the purpose of this thread is just for curiosity - whether anyone knows the reason for the strange step-to-every-track behaviour.
 
It looks to me that the head assembly is being asked to reinitialize it's home position in case that was the reason for the bad read. It approaches the home sensor slowly in order to maximize the chance of getting a reliable zero then goes immediately back to the track in question to try again.
 
paul said:
It looks to me that the head assembly is being asked to reinitialize it's home position in case that was the reason for the bad read. It approaches the home sensor slowly in order to maximize the chance of getting a reliable zero then goes immediately back to the track in question to try again.
Click to expand...

Hard to answer at this point. Run the utilities and then you'll have a better picture of what's going on. You know that drive has to be somewhere between 25 to 35 years old.
 
paul said:
It looks to me that the head assembly is being asked to reinitialize it's home position in case that was the reason for the bad read. It approaches the home sensor slowly in order to maximize the chance of getting a reliable zero then goes immediately back to the track in question to try again.
Click to expand...

Sounds like this is it. Thanks!

Agent Orange said:
Hard to answer at this point. Run the utilities and then you'll have a better picture of what's going on. You know that drive has to be somewhere between 25 to 35 years old.
Click to expand...

Thanks for the input, but I wasn't actually trying to fix anything. Again, this was just a general curiosity as to the behaviour of MFM drives when they encounter bad sectors.
 
So the drive took 3 hours to format, ended up with 1.2MB of bad sectors. All on head 3 (ie the 4th head - 0,1,2,3) - I might try tricking it into thinking it only had 3 heads and disable that surface, I might end up with a remotely reliable 15MB drive!
 
And, keeping in mind that your formatting software does not know or care that the drive uses a stepper motor. When the command to "home" is issued, it's the disk firmware that carries out that process on it's own terms. A drive with a voice coil actuator might do that same task nearly instantly.
 
I know this is an old thread, but for those that are interested I just wanted to share what mbbrutman suggested could be the reason for this behaviour on this youtube video

"I was a hard drive firmware engineer for a brief time, and that was for much later drives. So this is just a guess, but if for some reason you had read errors and possibly were not aware (or sure) of what track you were on, doing a full power seek back to track 0 might be a dangerous thing. Imagine the arm slamming against the stops at high speed. Walking it back a track at a time would be much safer."

It finally makes proper sense to me!
 
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