I ran a transfer of about 4 TB of files from my failing Mac to my new Mac. After 62 hours of direct connection transfer time (which seems inexplicable), I was told at the very end of the process that not all files could be transferred without telling me which ones I may have still stuck on my old Mac.
Is there anyway I can get a list of the failed transfer files or do I have to start the entire process over and hope it doesn't fail a 2nd time over the 3 day span of time?
You can compare contents by setting the source computer to Target Disk Mode, connecting both computers via cable or network, and compare contents using a tool like Beyond Compare (long free trial, more than enough for this job).
Open a terminal and execute:
diff -qr /Volumes/newharddrive /Volumes/oldharddrive
you have to change the names of the hard drives (or directories) according to your setup.
diff -qr /Volumes/newharddrive /Volumes/oldhardrive | grep ' differ'
is a variant to reduce the output clutter.
You can redirect the output from the terminal to a text file for easier reading:
diff -qr /Volumes/newharddrive /Volumes/oldhardrive >compare.txt
diff will take an excessively long time just to check if files exist, and not only that but it will actually fail to detect existing files as well, in the sense that it doesn't perform normalization of the filenames before comparing them. This means that a file "Namnlös" will be reported as missing just because APFS volume #1 did not represent "ö" the same way as APFS volume #2. This happened to me after using Migration Assistant followed by diff -qr.
rsync --dry-run can alleviate these problems and can turn a hour long process into a few seconds, in my experience.