Is it possible to detect the previous byte position on a hard drive after it has been overwritten?

Question

I keep hearing about these statements that "a hard drive is like a cassette tape because it is a magnetic medium". That got me thinking...

Back in the day when I used to play DJ as a young lad, I would record music off the radio while played as the announcer to each song. Sometimes I would mess up and would have to rerecord of the flubbed portion of the tape, and upon reviewing, there was a noticable quatlity difference. Later in life I would learn that quality difference came from the fact the of the previous recording still existed on the tape, at least enough to distort the sound. It's the same idea as bending a steel fork or spoon; if you bend it and then bend it back, you will always have a small indentation where the original bend had occured.

Does this characteristic exist in magnetic base hard drives, and if so, is it detectable? Please consider the scenario that it has only been overwritten once.

Answer 1

Does this characteristic exist in magnetic base hard drives, and if so, is it detectable?

No, because the poor technique of re-recording that you mention is completely avoided in digital data devices such as hard disk drives and magnetic tape. The differences between recording analog information versus digital information is not the major issue. It really boils down to the recording mechanism.

Magnetic recorders have at least two heads: an erase head and a write (or read/write) head. The purpose of the erase head is to lower the noise floor of the magnetic medium (the tape or disk) prior to writing a signal. When you backup the tape in your example, there is some length (the distance between the erase head and the write heads) of recorded tape that will not be affected by the erase head when you begin re-recording. You are relying solely on the write head to "erase" and overwrite the previous recording. At some point (depending on tape speed and the head distance), you will be recording on tape that has been erased by the erase head.

Digital magnetic media completely avoid this issue by always writing data in complete records. The area between the records is called an inter-record gap, or simply gap. Within that gap is a special area called the write splice. The erase & write heads must turn on or turn off only within these write splice areas, so as to never damage any existing recorded data (including the gap data immediately before and after each record). If you need an analogy, think of each record as a song (or recorded track) with gaps between each song. Instead of re-recording in the middle of the song, the proper technique would to always record the entire song starting within the gap. Note: the process of (physically) formatting a hard drive is the process of writing an address mark, ID record, (blank) data record and all necessary gaps for each sector on every track of the HDD. When a sector is "written", only the data record of the sector is rewritten. The address mark and ID record are never rewritten after the format.

Writing in chunks called records and having gaps and write splices is not done solely because of the distance between the erase & write heads. There is also the issue of proper reading of the digital information. Your tape recorder used amplitude modulation to record the audio onto magnetic tape. Digital data recorders use MFM or its variants, which rely on flux changes (not voltage levels) to indicate changes in bit state. Note that flux changes indicate bit reversal, not absolute bit value. So on the start of a read, the bit state is initialized to a zero, and "0"s will be clocked in until a flux change is read. Therefore the read head has to be turned on where there were zeros written, which is the gap that precedes every record.

Answer 2

While I am no expert, I believe the main difference is that cassette tape records an analog signal while hard-drives are digital. The analog signals on a tape would be much harder to perfectly zero out before rewriting, while the digital signal would be much simpler to get close to the "true" 0 or 1 signal.

Answer 3

Now, I believe that the issue here, is can magnetic traces still be recovered or have an affect on the data that you overwrite it with?

If it has been overwritten at least twice, it's very doubtful.

As Godric states, hard drive data is digital. The media it is recorded on is magnetic, but we are dealing with 0's and 1's. You can't have a 0.5. So the previously recorded data has no impact on the currently recorded data.

In your example, your dealing with ANALOG data. The previously recorded magnetic data wasn't erased, so it partially cancelled out or amplified your new data, since it wasn't virgin media.

Since we are dealing with digital data, the drive mechanism writes the chunk of data, and a checksum. That checksum is used to verify that no data integrity has been lost, and if necessary is used to help rebuild the data, if it has been damaged.

If during read, that Checksum doesn't match the freshly generated checksum of the read data, then the data is silently repaired using the existing checksum. So you can't get distorted data due to the previously recorded & overwritten data.

The issue also occurs with data recovery. Once the data is overwritten once or twice, it will be nigh impossible to recover... (See http://www.schollnick.net/wordpress/2009/04/oh-my-gosh-they-are-stealing-my-data/ ).