Hard drives and magnetism


Some advice on the HDD and magnetism matter needed.

Just how vulnerable are external hard drives to magnetism and magetic fields? I just recently bought two external WD drives and kept them on my guitar amp next to my coputer. There is a little space (plus the amp’s top side itself) between the 30W speaker and the drives, so they’re nowehere near touching. I’ve kept them like that for two weeks or something.

Should I be concerned? I haven’t noticed any problems so far anyway… Also, if once exposed to magnetic fields, will the drives be more susceptible to magnetism in the future or even gradually start “deteriorating” from the point of the first exposure?


This is a matter not much “clear” or “well defined”. Since hard disks use magnetic fields to set “0s” and “1s” in micro locations at the surface of the recording media is most probable that the data will be affected by a powerful magnetic field. The damage will be restricted to the data itself since no physical damage happen.

The real concerning is a powerful DC magnetic fields ( like the materials used in speakers ) that can traverse the metal shield of the hard disk. AC magnetic fields will be greatly “consumed” ( under the form of parasitic current in the surface of the metal ) by the metal shield of the hard disk and will not harm the data.

Conclusion: get the hard disks away from the back of the speakers. :bigsmile:

The magnetic flux is concentrated in the speaker coil area and outside of that the field is much weaker. Its likely that no harm has been done to the drives, however it’s “best practice” not to subject magnetic material to stray magnetic fields just as you would avoid knocking or dropping the drives.

Thanks for your answers!

The drive was placed on top of the amp (30W), but there’s a 10-15cm gap to the actual box and then about 7cm of wood/plastic/electronics (the top side of the box itself) before reaching the drives. I used a compass to check the strength of the magnetic field and discovered that it wasn’t alarming where the drives had been. Of course the needle changed direction, but it was still a little bit “loose”. Putting the compass behind (about 5-10cm) the coil made the needle freeze instantly and wouldn’t move even a little.

Also, the amp has not been turned on during the whole time and the power cord has not been connected to the wall socket.

Right now I’m checking the disks with Disk Checker and so far they seem ok. So, no degradation from now on then? If all’s clear now, nothing related will happen in the future?

One more thing: how come disks can get damaged when there are earth magnets inside them? They’re super strong and put very near the surface, right?

The field of the drive’s own magnets is mostly contained within the pole pieces.

If a drive was exposed to a data-destructive magnetic field, it would also damage the factory formatting, and modern drives cannot be low level formatted.

One saving grace though, to prevent bit fade at high densities, the magnetic material must be high coercivity

No completely sure I got all that… ^_^;

I’d be more concerned with the magnetic field of the speaker’s permanent magnet than the voice coil.

Best bet is to remove your external HDD from ANY location that could be a potential threat (including the 30w amp that has a stepdown transformer capable of radiating a magnetic field) such as heat, static discharge or being dropped/crushed.

Copy the data to another drive and check it for corruption. If it checks out then format the external drive and copy the data back there and place the drive in a safe location. :cool:

Anyone know of a way to shield the speaker/amp? I’ve got a bass rig that i’d lke to put a HD recording system on top of. i’d be amenable to purchasing some kind of alloy to separate the fields.

Google MuMetal for shielding material.