OK, here is what I am seeing. I used QBitTorrent to download the file Only.Murders.in.the.Building.S02E10.1080p.WEB.H264-CAKES[TGx]. The MD5 hash for the only.murders.in.the.building.s02e10.1080p.web.h264-cakes.mkv file is f63c9b25296bdbf4ae92640c5a65456a. If I use Finder to copy it onto my USB SSD drive, the MD5 for that file is d54e47ed0557d5239a3d656dd3831241, so clearly the files are different.
If I use cp or rsync to copy the local file onto my SSD drive it copies correctly with no corruption - they have same MD5 hash as the local file.
I opened a terminal window and use the cp command to create a duplicate copy of the original .mkv file which I called test.mkv, which has - as you would expect - the same MD5 hash as the original. If I copy this file to the SSD drive it copies correctly with no corruption. Let's try something else. I opened the original .mkv file in HexFiend and used the Save As option to duplicate it without editing it. Copying that duplicate file to the SSD works correctly too.
Copying the original .mkv file over to the SSD drive once more using Finder, I get exactly the same hash as the earlier corrupted copy, so it is being repeatably corrupted in exactly the same way. I renamed the original file and then used Finder to copy it and it was corrupted in exactly the same way.
So Finder is corrupting the file during the copy, but what does it actually look like? I wrote a program to compare the two files which produced this:
Size for both files is 1421335900 bytes
Matched 135139328 bytes from 1 to 135139329
Cleared 45056 bytes from 135139329 to 135184385
Matched 851968 bytes from 135184385 to 136036353
Cleared 40960 bytes from 136036353 to 136077313
Matched 82153472 bytes from 136077313 to 218230785
Cleared 4096 bytes from 218230785 to 218234881
Matched 28672 bytes from 218234881 to 218263553
Cleared 16384 bytes from 218263553 to 218279937
Matched 43192320 bytes from 218279937 to 261472257
Cleared 94208 bytes from 261472257 to 261566465
Matched 58478592 bytes from 261566465 to 320045057
Cleared 16384 bytes from 320045057 to 320061441
Matched 42364928 bytes from 320061441 to 362426369
Cleared 40960 bytes from 362426369 to 362467329
Matched 25534464 bytes from 362467329 to 388001793
Cleared 249856 bytes from 388001793 to 388251649
Matched 10924032 bytes from 388251649 to 399175681
Cleared 57344 bytes from 399175681 to 399233025
Matched 624123904 bytes from 399233025 to 1023356929
Cleared 20480 bytes from 1023356929 to 1023377409
Matched 397958492 bytes from 1023377409 to 1421335900 <EOF>
Processed 1421335900 bytes.
The pattern of the corruption is that every so often during the copy, large parts of the data are zeroed out in the target file. The file sizes are identical. But it doesn't happen on every file - just on the original file downloaded by qBitTorrent. You programmers might have noticed some hinky numbers there.
So the symptoms are:
- Using Finder to copy the file results in a corrupted target file (MD5 mismatch).
- Source and target file sizes are identical.
- Large sections of the target file have been zeroed out during the Finder copy.
- It is repeatable. If you copy the file using Finder once more it is corrupted identically (Same MD5 as the original copied corrupted file).
- If you duplicate the file (using cp), Finder will copy the duplicated file correctly.
- If you copy the file in a terminal window using cp or rsync it copies correctly.
If you are seeing this problem please report it to Apple so we can raise awareness of it and get it fixed.
