crazy strong magnet in 7200 RPM HDs?

[spotter]

os my thinkpad is at jamiphar's right now, he has it without the HD. One thing I noticed is that a screw is stuck strongly to part of the disk due to magnetism. Never seen a HD that emitted a strong magnetic field like this.

Is this normal?

[joester]

HD's have some serious magnets in them... go figure with magnetic disk storage...
Take one apart sometime (dead one of course). Pretty cool innards.

I only have a few 4200's sitting around, but they don't exhibit any external magnetic pull.

Joe

[spotter]

right, I know they have big magnets inside, I've never had one where it attracted metal objects to the HD.

[SteveS]

The magnets operate the “voice coil” which is what positions the actuator which the heads are mounted on. The term is a left over from early days of hard drives when the actuators were cylindrical and the coil was arranged like the voice coil in a loud speaker. Modern drives use a coil which is planer. The motion is horizontal rather than vertical, with the actuator pivoting on a ball bearing.

I don’t think the speed of the drive impacts the strength of the magnet. Rather it is the actuator design. The magnet is located to the side if the disks, and they must design it so minimal, if any of the field passes through the disk area.

Still, it is disconcerting when you can detect this strong if field outside of the drive. I have several 2.5” drives which I use to archive and back up various versions of the T40 machines I have, and I always wonder if it is safe to store them next to each other.

[mgo]

Still, it is disconcerting when you can detect this strong if field outside of the drive. I have several 2.5” drives which I use to archive and back up various versions of the T40 machines I have, and I always wonder if it is safe to store them next to each other.

Thinking back to my old radio school days, we were taught that a -moving- magnent would induce a voltage in a coil of wire, but if the magnent isn't moving, there is not any energy field created. And of course if the magnet were stationary and the coil was moving, the voltage would also be generated.

I would suppose that a strong magnent passed close to a platter would have a bad effect, but if the platter isn't seen as a "coil" there would be no spurious disturbance to the data.

Smarter users here are welcome to shoot holes in my theory...

[hart22]

Thinking back to my old radio school days, we were taught that a -moving- magnent would induce a voltage in a coil of wire, but if the magnent isn't moving, there is not any energy field created. And of course if the magnet were stationary and the coil was moving, the voltage would also be generated.

Sorry I had to jump in on this one...

A moving magnet induces a current in a stationary wire, provided that the wire is in the right orientation. Likewise, a charge moving through a magnetic field, B, experiences a force equal to F = qv x B, where q = charge magnitude and v = velocity, and x denotes the cross-product, which is a vector.

But this is only the effects of magnetic fields on currents and electrically charged particles.

A magnet defined as such always possesses a magnetic field, which always will affect nearby objects with magnetic dipole moments, i.e. the physical units used to store data in a HD. So a stationary magnet of relatively modest strength shouldn't go anywhere near a HD. Likewise if you have a pair of powerful speakers or even worse, a subwoofer, which all induce magnetic fields to move the diaphragm components, get that away from your laptop and desktop, or CRT monitor for that matter.

[carbon_unit]

The two magnets are placed so they magnify the magnetic field between them while suppressing any magnetic field around them. The movement is produced by inducing a current into the coil inside the magnetic field.
If they had a magnetic field around them they would corrupt the data on their own platters.
If you have a hard drive with an external magnetic field it has a defective magnet inside of it or the magnets have become misaligned.

[SteveS]

I have an answer from a good source. I was sitting next to design engineers from a hard drive company a few minutes ago (sorry – I can’t reveal the name of the company) at a SATA compliance test workshop. I questioned them about this.

Carbon unit is correct stating that there is a “keeper” around the voice coil to concentrate the field and contain most of it. According to these engineers, essentially none of the field exits in the direction of the disks, which are to the side of it. However, some of it does radiate through the cover of the drive. The drive is not defective if you experience this. However, it is not strong enough to penetrate far enough into an adjacent drive and erase anything on that drive’s disks. They ship drives in bulk to PC manufacturers side by side in the box, just wrapped in an ESD dissipating bag. So if this field were strong enough, it could damage the adjacent drive.

BTW, don’t think it is OK because these are new drives and don’t have any data on them yet. All modern drives have “index” tracks written on them by a precision writer during the manufacturing process. This operation requires very precise mechanical positioning, and laser interferometers are used to position the index track writer. Erasing the index track in a brand new drive would instantly destroy it.

[carbon_unit]

Ah ha! Pretty good for a guy who got a "D" in science class, huh? /me pats himself on the back
I knew most of the answers but I didn't know how to take a test. I got hung up on the tough questions and never got past them.
Now I am more interested and I know which questions to avoid.

[off1c3r]

Ah ha! Pretty good for a guy who got a "D" in science class, huh? /me pats himself on the back
I knew most of the answers but I didn't know how to take a test. I got hung up on the tough questions and never got past them.
Now I am more interested and I know which questions to avoid.

Lol... I did well in science but I didn't know/remember what was said here.. but I am gonna open up some drives tonight! Maybe I can make a levitating screw trick with an 80gig 7200rpm =)