Jerome Fine replies: Which is why I feel that using MD5 to verify a file after I read it back to a hard drive from the DVD is so essential. Actually, for files up to about 10 MB, I produce the MD5 directly from the DVD file. But I have found that with a 1 GB file, it takes less overall time to copy the file to a hard drive first. I don't exactly understand, but it is probably because the MD5 program has different buffer sizes than the COPY program - or because a single command copies all of the files from the DVD to the hard drive (so I don't need to execute an individual copy command for each file) whereas the MD5 program needs a separate command for each file. That would be helpful, but "long" is probably relative. Unless the DVD is actually experiencing slow bit rot faster than scratches and other external damage, probably the only status that would be helpful would be a percentage of the allowable damage before the errors are no longer recoverable. Can anyone suggest whether or not degradation due to bit rot can be distinguished from external damage? I keep my CD and DVD archival files inside their envelopes. I rarely mark the actual DVD media, however, I use a standard sheet of paper cut into 4 pieces that fits nicely inside the envelop to identify the DVD files on the media. My experience is that careful handling of CD and DVD media (limiting the contact to ONLY the edge of the media) probably prolongs the useful life of the files. I also attempt to keep media are burned - which that piece of paper is very helpful in preventing since I usually store 3 DVD media per envelope. Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.

I agree. On the other hand, it is painful to wait many minutes for each 1 GB file to be checked (produce the MD5 value) while still on the DVD and then manually check the next 1 GB file. So even if the time was the same to copy or check the file, it is far more convenient to just copy all the files on the DVD to the hard drive at which point I have also verified I can read the complete file. If I then check the file copied from the DVD media (reproduce the MD5 value), I am confident that the file on the DVD is completely valid. Interestingly, reproducing the MD5 value the read speed of the file) and it takes me about the same time to manually scan the original saved MD5 value and the new value and compare them. And I just discovered that the length of the MD5 value is 128 bits (32 hex digits - I had though it was that long but somehow the 32 hex digits always became 32 bits when I though about the size) which should be more than sufficient to confirm that the file I have read with the same size as the original is IDENTICAL to the original. I remember that Tim Shoppa used MD5 values for BZ2 files that were downloaded. Since it is rare for downloads to be corrupted, MD5 is probably the most efficient way to verify that the downloaded file is identical. I agree that it would be better to use some methods to restore the archived file if corruption takes place, but yearly testing should minimize that problem. Plus with a monthly backup image, the file I want to recover is most often on more than a single backup image. Sincerely yours, Jerome Fine

Jerome Fine replies: It seems that there are md5 programs which are more flexible than the single file (at a time) use version that I am using. Any help possible on a link to an enhanced version? Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.