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There is no reason for the DoD spec other than paranoia.

Check out this article from Seagate Recovery Services

It has been suggested that an electron microscope could be used to read and interpret any patterns that were not fully overwritten by the process. Theoretically this can be done - but in practice it is little more than a myth.

If data could be recovered at the rate of 1 bit per second - this process would take 9,259 days (or over 25 years) to recover 100 MB of information. This is assuming that you could read back and interpret each bit correctly, for example on data that has never been overwritten. If you are trying to read "traces" of data that were previously written there, in the most likely scenario you may be able to correctly recover, interpret and identify 30-40 percent of the signals.

THAT DOES NOT MEAN YOU WOULD RECOVER 30-40% OF THE DATA - BUT ONLY 30-40% OF THE INDIVIDUAL BITS IN EVERY CHARACTER.

A "10101011" pattern may come back as "?010?01?" and every single character on the drive would be scrambled in a similar manner. The mathematical probability of decrypting such a puzzle into usable data is infinitesimal.

It could be claimed that data can be recovered from any drive in the world with a guaranteed success rate of 50% "at the bit level". This sounds interesting until you consider that if you overwrote the entire surface of the drive with either all "0" or all "1" and since the original drive contained nothing but patterns of binary ones and zeros - half the bits would be correct - but obviously no data could be recovered.

In conclusion, overwritten data cannot be read back or recovered by any current disk drive technology or laboratory technique.

http://www.actionfront.com/ts_dataremoval.aspx

Smashing the drive is likely less effective than overwriting and overwriting is almost impossible to recover from. Even with an STM!

If anybody wants some fairly good information, then check out this site:
http://www.actionfront.com/ts_whitepaper.aspx

They have white papers on the subject. If you disregard the glorification the company allows themselves at the end of each paper, it gives some insights about what the data recovery services (usually) aim for and some basic stuff about hard drives.

This paper provides a great explanation of the current state of the data recovery industry. How modern hard drives work, how they fail, how they can be recovered, myths and realities.

[PDF] Recovering Unrecoverable Data

Unless the company has made great advances in the product they advertise at the end of the paper, you can be sure that two passes are more than enough to prevent anyone from recovering your data. Intelligence agencies are more likely to kidnap and torture you than invest the extraordinary time and money to get your bits back.

"the FBI can see data that has been overwritten 12 times"

Bull. Shit.

If the data has been overwritten (actually overwritten, not just "deleted" or disk format) there's not a company/organisation/indivitual in the world that can read the data that used to be stored there.

Granted, an on-track overwrite will in most cases leave residual off-track magnetic trace that could be recovered using exotic forensic techniques, but this can be extremely difficult and highly unreliable - especially for newer HDD's using drive optimization techniques to optimize each individual drive. To successfully recover user-readable data from a drive where the sectors have been overwritten would take a huge amount of man-hours and effort.

For a great introduction to the topic read the paper "Recovering Unrecoverable Data - The Need for Drive-Independent Data Recovery" written by Charles H. Sobey.

I'm old and interested enough to know what REALLY happened through the history:

First, as taught in any school book and computer manual through history (see Apple, Amiga, Microsoft, Commodore): 1024 bytes = 1Kilobyte, 1024 Kilobyte = 1 Megabyte etc. because the computer could only calculate in exponents of 2 (1 and 0) and 20MB (20480 kilobyte) was about the largest size hard drive you could get.

A Kilobyte is 1024 (2^10) bytes. A Megabyte is 1024 Kilobytes or 1,048,576 bytes (2^20) and a Gigabyte is 1024 Megabytes or 1,073,741,824 bytes (2^30), some background data on what comes next can also be found here: http://www.actionfront.com/hdtech1.html

As soon as the hard drives began to hit 1000MB and the price/capacity got obfuscated as well as the need to count with cylinders, headers and sectors and calculate your partitions based on that (yes, we used to do that for minimal loss of capacity and performance), people started to forget about the extra 24MB on the hard disk, because now it was not 1000MB but 1 Gig and of course the manufacturers started using 1000MB as 1GB etc. And then we could avoid talking about Int13h for addressing hard disks, we got translating BIOS'es and everybody forgot that we used to have 1024 bytes in a kilobyte. As you know, RAM still uses the correct count because we're still not mainstream on >4G of RAM but I imagine manufacturers will start 'forgetting' too and labeling 1000MB as 1GB

I have commonly heard it said that overwritten data can be recovered, so I went Googling for a rebuttal to this argument. Turns out, you appear to be right! Recovering of overwritten data is largely a myth. /me continues to use good ole' shred.

The faster a disk spins, the more disk surface is exposed to the magnetic field used to write to the drive, so the less storage you have.
You got it completely wrong here (and someone even modded this "5 informative"!!).

From actionfront.com: To get more data on a track, the spacing between each bit in the down-track direction must decrease. The data density in this direction, also called the linear density, is measured in thousands of bits per inch (kbpi). Similarly, the track density across the disk is measured in thousands of tracks per inch (ktpi). Areal density is the metric used to quantify the impressive growth in HDD data storage capacity. It is the product of bpi and tpi, which reflects the amount of user data that can be stored reliably in one square unit of area on the disk surface. It is now measured in gigabits per square inch (Gb/in2). For decades, the compound annual growth rate (CAGR) of areal density was about 30%. With the introduction of MR (magneto-resistance) head technology around 1990-1991, the rate increased to 60%. When GMR (giant magnet-resistance) heads were introduced in the late 90's, the CAGR temporarily increased to over 100%, during which time there was an increase in the number of companies that exited the industry or merged. The pace of areal density growth is now slowing and should settle somewhere between the historical rates of 30 and 60%.

See also Kryder's law: Kryder's Law is essentially Moore's Law for storage. But the density of information on hard drives has been growing at an even faster rate, increasing by a factor of 1000 in 10.5 years, which corresponds to a doubling roughly every 13 months.

As you see, nowhere does it say "disk speed". Truth is, as area density increased, disk speed increased as well (from 5000 to 7500 to 10000). And at the same time, data transfer thru the disk head increased as well (math teaches that... if bits per inch increase, and rotation speed increase, then data transfer must increase as well).

There are still ways of retrieving data from smashed or damages hard drives. Take a look... here are a couple of the companies that are doing it:

Data Recovery Specalist
Cherry Systems Data Recovery
Action Front Data Recovery Labs
DTI Data Recovery

There are lots of places doing it... including from fire and water damage.