Obviously they’re read remotely magnetically by their very nature. According to the Wik, the distance between the read/write head and the platter–the word for that gap, as I just learned, is the flying height, is 3 nm (which I also just learned, and find amazing–the diameter of a helium atom, is, again, according to the Wik, about 0.1 nm; ribosome 20 nm; regular old light wavelength is 400-800 nm.)
Anyway, an extraordinary range of remote sensors exist, eg, here.
So, what could spies do with that to get a read on a disk?
*No doubt classified terrain maps for cruise missiles and whatnot use real-time or near-real time equipment for extraordinarily detailed and updated data.
It would be extraordinarily difficult. Even a pure dipole (the simplest magnetic configuration known to science) has a field that falls off as 1/r cubed, and a magnetic storage device is composed of a great many dipoles arranged in different ways, which makes it an extremely high-order multipole, with a field that falls off correspondingly quickly. In practice, the size of the portion of the disc that encodes a single bit is also going to give you the rough distance at which it’s practical to read bits.
Sorry to hi-jack so soon, and I can’t answer that question. But this may be related.
The hi-jack is in related to the reader head and air-density.
Hard drives work OK to about 7000 feet. That’s what airliners are pressurized to.
They also work fine at higher altitudes provided it’s a static machine, a desktop is OK. A laptop or netbook, not so much.
I found this out the hard way when my netbooks continued to be erratic . Drove me nuts. My house is at 11,200 feet.
Not enough air under the reader head, and it kept bouncing off the disk if the machine was moved. Typing could do it. Caused the weirdest problems. I could not duplicate the errors. You just never knew what would happen. I never made the connection.
I switched over to Solid State Drives, and all is good. (I think the SDMB helped me with this)
The basic problem is that the bits are too close together on the disk to be read at a distance.
A good equivalent is that you can read the words on a sheet of paper. You can also see things on the other end of a football field. Put the sheet of paper at the other end of the football field, though, and you have no chance of reading the words on it.
Disk drives also have multiple platters, so instead of trying to read words on a single sheet of paper, try reading words on 8 different sheets of transparent plastic stacked one on top of another, and you want only the 4th word on sheet 7, again, reading from the other end of the football field, using only your eyes.
Ain’t gonna happen.
And the football field analogy only gets you a few inches away from the platter. If you want to read the disk from, say, outside your house, now you need to take those transparency sheets and put them oh, say, somewhere in the vicinity of the Moon. Sure, your eyes can look up and see the moon, but at that distance you aren’t even going to be able to detect that the sheets are there, let alone read any words off of them.