Is copy and paste for large files exact?

philjayj · 29 Jul 2023 at 15:47

I've tried looking online for an answer but couldn't find anything. Basically, when copying a load of mkv files from one drive to another, is the copied file an exact copy? Lets say I copy/paste a 50GB file, is there any chance of the copy not being the same as the original?

Cheers

throwaway4372 · 29 Jul 2023 at 15:55

It's not impossible that the copy could mess up somehow, but generally a copy paste is exact yeah.

If you want to be certain that files are identical you can generate a hash.

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TheVoice · 30 Jul 2023 at 16:41

FastCopy is a good tool to use, it's generally faster at copying/moving files than Windows is and it has the option to verify.

Murphy · 30 Jul 2023 at 18:32

The chances are very low as Windows does Cyclic redundancy checking (CRC) when copying files, the exact internals, in fact how CRC works, are beyond me.

From experience I've had files that have failed to copy partway through with a CRC error so I'm guessing Windows compares the CRC values on chunks of the file, could be 512k as that's the default NTFS block size.

Semple · 30 Jul 2023 at 19:24

Bit of a strange question. Are you just concerned about data corruption when copying large amounts of data? As mentioned above I'd just generate a hash of each file and then do a check after the copy to confirm all is intact.

If the hashes match, then the copy is a perfect match.

philjayj · 30 Jul 2023 at 20:12

Semple said:
Bit of a strange question. Are you just concerned about data corruption when copying large amounts of data? As mentioned above I'd just generate a hash of each file and then do a check after the copy to confirm all is intact.

If the hashes match, then the copy is a perfect match.

Yes, it's mainly data corruption I'm worried about. I'm currently copying about 30TB of mkv files to LTO8 (just using LTFS, not Veeam or anything like that). It would be a disaster to recover a file and discover it's corrupt.