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You are here: MacNN Forums > Software - Troubleshooting and Discussion > macOS > Verification of copied files: does a successful copy mean a file's not corrupt?

Verification of copied files: does a successful copy mean a file's not corrupt?
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BoulderDash
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Jan 16, 2011, 11:21 PM
 
Hey there,
I recently had a 2TB external hard drive start to fail (very slow transfer speeds, unable to eject it from the Desktop, unable to copy some files to a host iMac, failing all disk tests, etc.). On this 2TB external hard drive were 165 movies that I'd created with iMovie, and that I was looking to copy to a host iMac.

Knowing that this 2TB external hard drive was on its last legs, I began to copy the 165 movies to a host iMac last night. After 20-30 movie files, I would get an error stating that there was a problem with a certain movie and the copy would cease and freeze everything. I would then restart the iMac and 2TB external hard drive and begin the process with the remaining files (not including whichever file had caused the error). Once again, a different file would trigger an error and I would restart the process once more.

Of the 165 movies, I'm happy to say that I was able to transfer 161 of them off of the 2TB external hard drive and onto a host iMac. The remaining 4 always cause a copy error. This leads me to ask the following question to people with more knowledge of how the Mac OS X system copy works:
  • If a file is successfully transferred from a failing disk to a non-failing disk, is it safe to assume that the file is not corrupt and has been transferred intact?

I'm curious about this, because I'd like to know about the integrity of the 161 movie files that were successfully copied to a new disk. I do not want to have to actually view all 20-30 minutes of each movie to know whether or not they are corrupt. Does a successful copy to another disk mean that the Mac OS X system verified the stability and integrity of a file before it deemed it successfully copied (and caused errors on files that were not deemed to be in good standing)?

Of course, I'm hoping the answer is "Yes, if they copied you're all good and your files have been saved (now, do better backups!)"... but I don't know if it's that simple.

Any insight into this would be very appreciated. Thanks so much!
BD
     
shifuimam
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Jan 17, 2011, 02:24 AM
 
There's no real guarantee that it copied over without corruption.

I'd recommend trying a recopy using grsync, which is a GUI frontend for the rsync UNIX utility. This will let you do checksumming and all kinds of fun things to verify your files, plus it will automatically bypass anything it can't copy and create a log of what failed.
     
OreoCookie
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Jan 17, 2011, 03:47 AM
 
There is no guarantee, especially if the source is not trustworthy. You can be very, very sure if you create a SHA1 checksum of the files and compare them. Since it's not that many files, you can do that without too much of a hassle.
I don't suffer from insanity, I enjoy every minute of it.
     
BoulderDash  (op)
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Jan 21, 2011, 10:36 AM
 
Hey there,
Thanks so much for the advice. I really liked using Grsync and reading about SHA-1 checksums.

See you,
BD
     
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Jan 21, 2011, 03:24 PM
 
Verification just means the copy is complete. You could have completely copied a corrupt file for sure.
"Well done is better than well said." -BF
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