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To: rupert1 who wrote (43436)	1/13/1999 1:16:00 PM
From: TigerPaw	Read Replies (1) \| Respond to of 97611

Is it removed, distorted or suppressed? (Erased Data One analogy of recovering erased data can be shown by holding two sheets of copier paper (with writing) up to the light, one directly in front of the other. You can still see some of the bottom sheet through the top. Another analogy is to take a broad magic marker and draw a straight line on a piece of paper. Now take a different colored marker and quickly draw over the same line. Notice that some of the original color can usually be seen at one edge of the line or the other. There are magnetic pickups which are more sensitive and precise than the usual disk read heads. These can look at the edges of the tracks and also detect residual magnetism to recover written over data. A security erase usually involves writing random patterns over and over and over hundreds of times to mess up these effects. TP

To: rupert1 who wrote (43436)	1/13/1999 1:27:00 PM
From: Night Writer	Read Replies (2) \| Respond to of 97611

victor, I had the same problem one time. Some models I had written on LOTUS were deleted by accident by another person I shared the computer with. A tech recalled the files. There had been some file damage, but it was better then starting from scratch. The tech advised me of the following. Once you write over a program, you can't recover it. Once a disk is formated, the data is gone. Also there are some security programs called shredders that also prevent deleted data from being recalled. The files he recovered had been partly written over after they were deleted. This was also on the old DOS OS. I'm sure rudedog will be much more informative on this problem. NW

To: rupert1 who wrote (43436)	1/13/1999 6:34:00 PM
From: rudedog	Read Replies (3) \| Respond to of 97611

TigerPaw's response was (as usual) first rate. The magnetic information on the disk is read and translated to digital form in the drive electronics on a standard disk. But if the platter is removed, and a different head, with different electronics, is used to read the tracks, then the analog information about previous write operations can usually be recovered. I worked with a military team who were able to recover 5 layers of data from a drive using this technique, kind of like uncovering the seven cities of troy... These techniques can usually recover even data that has been 'shredded' using commercial programs. Standard formatting creates disk 'blocks' which are containers for 512 bytes of data (other block sizes are possible but 512 is standard for PC systems). Each block also contains address and checksum information so that if there are failures in the magnetic medium, the data can still be read by performing a mathematical operation on the readable data, then applying the error correction codes. Some systems have track level error correction. Some SCSI drives also automatically map 'bad blocks' to new good blocks, and the user never knows that a block has gone bad. CPQ uses this as a part of their pre-failure warranty - if the rate of generation of bad blocks is high, or the number of blocks is over a threshold, then cpq will replace the drive even though it is still fully functional, thus avoiding data loss. When a file is deleted, the file itself is not changed - none of its blocks are altered in any way. Instead, directory information about the file is changed, to make the 'containers' available for new contents. This is a complex and fascinating topic, if you are interested we can continue in more depth via PM.