Wednesday, 26 May 2010

magnetic fields

back on the 15th I was up "north" visiting a friend for a wedding (naturally I seemed to be sucked into that as photographer). I took some with digital and some with 35mm film. I had the 35mm film scanned by Photocontinental who uses a Noritsu scanner (expect to see comparisons and details on that soon).

However in the evening while I was downloading the digital images of the day onto my laptop my computer started reporting all manner of errors; Adobe can't find this font, C:\blah\blah is missing ... the list goes on

Quickly I realised that my computer had been sat inadvertently on the subwoofer of a logitec 5.1 system and went "oh shit"

On shutting down and restarting I was presented with the black screen of no motion and a disk that would not boot.

I restored a "snapshot" (quick plug for driveSnapshot which has saved my bacon on many occasions) and all seemed well ... for a few days, then errors started to creep back in and the computer would not boot properly.

I tried re-partitioning the drive; formatting running Chkdsk and then restoring a snapshot ... but still no go after a short while (few days) the error returned.

I have no way of knowing how to do a "low level format" as we once did with the tools available to us such as "Norton Utilities" or "PC Tools" so with the underlying format seemingly somehow compromised I have decided to cough up for a new drive ... this one was only 4 months old!

I am unable to find reliable information on the WWW about the effects of magnetic fields on hard disks, but if this situation is not a coincidence then it does suggest that the platter has been degaussed by the operation of the drive in close proximity to the strong magnet of the subwoofer.

I've no way to test the drive (say, check for analog signal to noise ratio on the sector reads) but if anyone knows I'd love to re-establish that disk as usable.


LensBubbles said...

That's interesting. I thought most drives were sealed against magnetic interference from external sources. One way to confirm this is to place a know good drive, that does not contain useful data, and leave it on a known strong magnetic field, such as a subwoofer for a day or so, and see what happens.

obakesan said...

leaving it sitting would not be a full or complete test, one would need to also operate the drive and see what effect the magnetic field had on the read write process and on other things by its induction effects.

but yes, I'd like to test this further

Noons said...

A static electric field such as a big magnet in a subwoofer won't necessarily wipe out a disk.

When it comes to wiping magnetic registration, the wiping has to be done with a varying field, not a static one. This is why the erase head in magnetic recording works with a high frequency alternating current - around 60KHz for sound recordings.

The big magnet can and will cause permanent magnetization of steel surfaces.

What likely happened is that the strong field caused some part inside the puter to become magnetized (usually a heat shield or even a large screw) and that now causes any moving magnetic field (a recorded portion of the disk) to go blank when it passes through that field.

Hence why problem comes back after a while and having restored.

Might be worth taking it to a service shop to confirm if they can demag the lot, without any disk drive inside it.

Otherwise you might find the new disk also "developing problems" after a while.

Not saying it's that, but something to kep in mind.

obakesan said...

Noons, thats a cunning theory. Looking around the laptop (IBM Thinkpad x31) there is precious little metal or screw around the area.

The "test" of your theory will be if this new drive starts to dingle up on me ... perhaps I'll have to resort to degaussing the screws around it ... wonder if a tape head demagnetizer would do the trick?

Noons said...

Man, I remember those! Likely it would but you'd have to have access to the guts of the lappie as it only works at close range.

Old TVs and CRT monitors used to have a demag loop around the back of the screen that kicked in when you turned them off: those worked really well for big objects but nowadays are hard to come by. If memory doesn't fail me, they used 90V 100Hz ac or thereabouts.

Anyways: it probably was something else. I doubt the big magnet alone would have done it: it's a constant field and those do not cause demagnetization of high frequency recordings. You'd have to move the lappie over it very fast to get a problem.

Had a similar problem with an older PC a while ago, killed all the disks I gave it. Once demagnetized, no more problems! But nowadays this is probably a non-event.

Demagnetization happens when a fast changing field moves over a stationary object or a fast object moves through a static field. Ie: you either vary the magnet (ac current or movement) or the object (movement across the field).

Oh, another thought: did you have the lappie on while on top of the speaker? That'd have done it: disk rotating at high speed through constant high mag field, instant wipe effect!

obakesan said...

I'm a big (pre Lenovo) Thinkpad fan, I have 2 x31's running as well as a x20 and even my ancient 770ED is still pulling service as my print server / NAS

I am no stranger to internal surgery on the Thinkpads, as all have been 'operated' on to keep odds and ends running for really next to nothing.

Perhaps my IBM family deserves a blog post?

I love the x31 as it has built in firewire (for my Coolscan) and a native IDE type slot for CF media.

Its my mobile image center

obakesan said...

Ohh ... and yes, it was turned on and downloading images ... only for a short time before I went "ahhh" and moved it

Noons said...

That woulda done the damage, I'm afraid... On the other hand it's good news: means prolly nothing wrong with the lappie.

Definitely do a post on the Thinkpads: bound to be other folks out there who use older stuff and want to know what can be done with them. They show up for peanuts at swap meets and local fairs.

Still got my old Pentium-III Asus with a meagre 350M main mem. Mostly because it's the only way I've found to access the Nokia and the Palm via the infra-red port: the new infra-red USB-based adapters just plain won't work. Ah, technology...

Mind you: I'm soooo tempted to get an ipad! Dying to see what the Android equivalents will do.

Gotta get around to finally setup a wi-fi router at home: I've got wired G-bit E'net to most rooms, but the kids are demanding wi-fi for their i-touch's and other gizmos they want to use.