I’ve been asked a few times over the past weeks about making multiple copies of disk images. Specifically, if I were to make a copy of a copy of a disk image, would the “quality” degrade? The short answer is no. It boils down to the idea of copying information from a digital format (as opposed to an analog format). Let’s say I write down the following string of ones and zeros on a 3×5 card:
101101
Now, if I take out another 3×5 card and copy over (by hand) the same string of ones and zeros, I now have two cards with the string:
101101
Finally, if I took out a third 3×5 card and copied yet again (by hand) the same string (from the second card) I would have three cards with the string:
101101
Assuming I had good handwriting, then each copy would be legible and have the same string of ones and zeros. I could continue on indefinitely in this manner, and each time the new 3×5 card would not suffer in “quality”.
However, instead of copying (by hand) the string of ones and zeros, I could have photocopied the first 3×5 card. This would yield a result of one 3×5 card, and a (slightly) degraded copy. If I then photocopied the copy, I would get a further degraded (third) copy.
So, copying images (or any digital information) verbatim (i.e. using a lossless transformation process) doesn’t degrade the quality of the information, read a “one” (from the source) write a “one” (to the destination). Where you might run into trouble is if the source or destination media has (physical) errors. So it’s always a good idea to verify your media before imaging. It’s also a good idea to use a tool that tells you (and even better if the tool logs) errors that it encounters during the imaging process.
